By the end of this lesson you will be able to
- Explain what a file and a folder are, using an everyday comparison.
- Choose clear file names that avoid characters that cause trouble.
- Read a file extension and say what kind of file it probably is and which program opens it.
- Show file extensions and open the Properties of a file on Ubuntu and Windows.
Everything you save on a computer, from a letter to a photo to a song, is kept as a file, and files are kept in folders. If you understand how they are named and grouped, you can always find your work again, share it without confusion and avoid the sinking feeling of a file that will not open. This lesson gives you the vocabulary and habits that every later lesson relies on.
A file is a named container of information
A File is a collection of information saved under one name. The information might be the words of a letter, the dots of a photograph, the sound of a song or the instructions of a program. To the computer they are all the same thing: a long run of bytes kept on Storage with a name attached.
A folder (also called a Directory) is a named place that holds files and other folders. Think of a paper filing cabinet. Each sheet of paper is a file. Each cardboard folder is a folder, and a drawer is a folder that holds folders. Unlike paper, a computer folder can hold as many items as the drive has room for, and it never gets heavy.
The system that remembers which bytes belong to which file name and where they sit on the drive is called the File system. You never have to talk to it directly. You use a program called a file manager: Files on Ubuntu and File Explorer on Windows.
- Marker 1: A shortcut to a folder you use often
- Marker 2: A folder: it holds other items
- Marker 3: A file: it holds information
- Marker 4: The extension (.jpg) follows the final dot
Read this figure as text
A simulated file manager window listing a folder called Letters that contains three files with different icons.
A file manager window titled Documents. The left sidebar lists Home, Documents, Pictures and Downloads, with Documents highlighted (marker 1). The main area lists one folder, Letters (marker 2), and three files: Budget-2026.xlsx, a spreadsheet; Photo-market.jpg, an image; and Notes.txt, a plain text file (marker 3). The icon of each item hints at its type. The file names end in a dot and a few letters: those letters are the extension (marker 4).
Choosing good file names
A file name is how you and the computer refer to the file. A good name tells you what is inside without opening it. Compare scan0001.pdf with Electricity-bill-March-2026.pdf. Six months from now only one of them makes sense.
- Be specific. Say what it is, who it is for and when:
CV-Amina-2026.docx. - Keep dates sortable. Write the year first, then month, then day, for example
2026-03-14-meeting-notes.docx. Then an alphabetical list is also a list in date order. - Use hyphens or underscores instead of lots of spaces. Spaces work on both systems, but hyphens are easier when names are used in web addresses and commands.
- Avoid special characters. Windows refuses
\ / : * ? " < > |in a name. Linux refuses only/. Avoid all of them so that a file can travel between the two systems without a rename. - Do not rely on capital letters. Ubuntu treats
Report.docxandreport.docxas two different files. Windows treats them as the same name. Using one style throughout avoids confusion.
Extensions: the clue after the dot
The last few letters of a file name, after the final dot, are the extension. The extension is a label telling the computer, and you, what kind of information the file holds and which program should open it. Budget-2026.xlsx ends in .xlsx, so it is a spreadsheet and should open in a spreadsheet program such as LibreOffice Calc or Excel.
| Extension | Kind of file | Usually opened with |
|---|---|---|
.txt | Plain text with no formatting | Text Editor (Ubuntu), Notepad (Windows) |
.docx, .odt | Word-processor document (Microsoft Word, OpenDocument) | LibreOffice Writer, Word |
.xlsx, .ods | Spreadsheet | LibreOffice Calc, Excel |
.pdf | A PDF: a fixed-layout document for reading and printing | A browser, Document Viewer, Edge |
.jpg, .jpeg, .png | Pictures (photos are usually JPG; screenshots PNG) | Image Viewer, Photos app |
.mp3 | Music or other audio | Music or Rhythmbox, Media Player |
.mp4 | Video | Videos, Media Player |
.zip | An Archive holding other files in one package | Files, File Explorer |
.exe | A Windows program (never open one you were not expecting) | Windows only |
Budget-2026Base nameYou choose it.Final dotSeparatorxlsxExtensionTells the typeRead this figure as text
The file name Budget-2026.xlsx broken into two labelled parts: the name you choose and the extension.
The file name Budget-2026.xlsx has two parts. Budget-2026 is the base name: you choose this to describe the content. The dot is the separator, and the last dot in the name counts. xlsx is the extension, which tells the computer the file is a spreadsheet and which program should open it.
- Contents: a photo
- Label says: JPG
- Photo viewer opens it
Label and contents agree.
- Contents: still a photo
- Label says: PDF
- PDF reader shows an error
Label and contents disagree.
Read this figure as text
A comparison of a correctly labelled photo file and a photo that was wrongly renamed to end in pdf.
Left column, Correct: the file holiday.jpg really holds a photo, its extension says jpg, so the Photos app opens it. Right column, Wrongly renamed: the same photo was renamed holiday.pdf by hand. Its contents are still a photo, but the label says PDF, so a PDF reader cannot read it and shows an error. The fix is to rename it back to holiday.jpg, or open it in an image program and export it as a real PDF.
Showing extensions and file properties
Ubuntu shows extensions by default. Windows 11 hides the extensions of known types until you ask for them. Turning them on is a good habit because it lets you see what a file really is. Every file also has Properties: a small panel with its type, size, location and dates.
See extensions and properties
On Ubuntu 24.04 (Files)
Step 1: Open Files
Click the Files icon (a folder) in the dock, then click Documents or Downloads in the sidebar.
You should see: You see a list or grid of your files, each with its full name including the extension.
Step 2: Switch to the list view
Use the view buttons in the toolbar at the top right and choose the list view, which shows Size and Modified columns.
You should see: Each file sits on one line with its size and date.
Step 3: Open Properties
Right-click a file and choose Properties.
You should see: A window shows Type, Size, Parent Folder and Modified, with tabs for Basic, Permissions and sometimes Details.
On Windows 11 (File Explorer)
Step 1: Open File Explorer
Press the Windows key and E together, or click the folder icon on the taskbar. Click Documents in the left pane.
You should see: A list of files and folders appears.
Step 2: Show the extensions
In the toolbar choose View, then Show, then tick File name extensions.
You should see: Names that looked like
Budgetnow readBudget.xlsx.Step 3: Open Properties
Click a file once and press Alt and Enter together, or right-click it and choose Properties.
You should see: A box shows Type of file, Opens with, Location, Size and dates.
Match each file name to the type of file it is and the program that would open it. This is a simulation: nothing on your computer is touched.
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Common mistakes
✖ Instead of: Renaming a file's extension to change its type.
✔ Do this: Open the file in the right program and use Save As or Export to create a real file of the new type.
✖ Instead of: Saving everything as
Document1orscan0001.✔ Do this: Use names that say what the file is and when it is from. You will thank yourself later.
✖ Instead of: Leaving Windows extensions hidden.
✔ Do this: Turn on File name extensions so that tricks like
invoice.pdf.exeare visible.
If something goes wrong
- A file will not open, or Windows says it cannot open this file.
- Check the extension. If it is wrong or missing, the computer does not know what the file is. If it is right, you may not have a program for that type installed: search App Center (Ubuntu) or the Microsoft Store (Windows).
- I cannot see extensions on Windows.
- In File Explorer choose View, then Show, then File name extensions. The wording can vary slightly between updates, but the option always sits in the View menu.
- I get an error when I rename a file.
- The name may contain a forbidden character such as
/or:, or the name may already be used in that folder. Remove the character or choose another name.
Key terms
- File
- A named chunk of stored data, such as a document, photo or song.
- Folder
- A container for organising files and other folders.
- File extension
- The letters after the last dot in a file name that show its type.
- File system
- The way a drive organises and keeps track of files and folders.
Practise it for real
Reading is not the same as being able to do it. Complete these on your own computer, then tick only what you have actually done. These ticks are your own honest record: nothing checks them for you.
Inspect three real files
On your laptop, open your file manager and go to Documents or Downloads. Turn on file extensions if you use Windows. Choose three files of different kinds. For each one, write down its full name, its extension, what kind of file you think it is, and its size from Properties.
Challenge: design a naming habit
Imagine you save a monthly electricity bill, a CV and some photos from a family event. On paper, invent a naming pattern for each that includes what it is, a date that sorts correctly and no special characters. Then write down three names that break your rules and explain what would go wrong with each.
Knowledge check
6 questions. You need 80% to pass. Mistakes here are part of learning, and you can retry as often as you like.
Summary
- A file holds information under a name; a folder holds files and other folders.
- Good names say what and when, sort by date and avoid special characters.
- The extension after the final dot tells the computer and you the type of file.
- Renaming an extension does not convert a file. Use Save As or Export.
- Show extensions on Windows so that disguised file names are easy to spot.
Checklist: can you do these?
Marking a lesson as read does not prove you can do it. Skills count as demonstrated when you pass the quiz (not yet) and complete the practical tasks (not yet).
Related lessons
References and further reading
Official sources for details that can change between versions.
- Files: Ubuntu Desktop guide (Ubuntu)
- Windows File Explorer: support (Microsoft)