10 computer file terms explained for beginners

File tools use words that can make a simple job confusing, such as format, extension and compression.

The examples use Squoosh's sample photo and fictional files in Photopea and draw.io. The explanations follow the linked documentation.

1. Read the file extension

An extension is the ending after the last dot, such as .png or .pdf.

If the ending is hidden, show it before checking the file:

• Windows 11 (opens in a new tab): in File Explorer, choose View > Show > File name extensions.
• Mac (opens in a new tab): in Finder, choose Finder > Settings > Advanced, then select Show all filename extensions.

1. Look at the full filename before opening it.
2. If an app asks for a different format, use an export or conversion tool. Renaming .png to .jpg does not convert the contents.

2. Check the file format

The format is how the contents are stored. An extension is a clue to it.

1. Read the format requested by the receiving app.
2. In Photopea, choose File → Export as and select that format.

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Photopea Save for web dialog with PNG selected and a fictional poster preview

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3. Compare file sizes

Bytes, kB and MB describe how much data a file contains.

1. Read an upload limit before choosing a file.
2. Compare the file's size with that limit. If it is too large, export a smaller copy.

Squoosh's sample showed 2.79 MB originally and 862 kB after compression. The smaller figure is the amount of file data, not the picture's width.

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Squoosh sample showing the original size of 2.79 MB and compressed size of 862 kB

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4. Count the pixels

A raster image is a grid of picture elements called pixels.

1. Open Image → Image Size in Photopea.
2. Read Width and Height with the unit set to Pixels.

1280 × 720 means 1280 pixels across and 720 down.

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Photopea Image Size dialog showing 1280 by 720 pixels, DPI 72 and Resample enabled

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5. Distinguish dimensions from print resolution

Pixel dimensions give the image’s width and height in pixels. DPI means dots per inch; the image-size dialog uses it as a print-density setting.

1. For a web upload, check its required pixel dimensions.
2. For printing, ask the printer for the required size and resolution. A higher DPI setting does not restore detail missing from the image.

6. Resize the whole image

Resizing changes its dimensions.

1. In Squoosh, turn on Resize.
2. Keep Maintain aspect ratio checked and change the width. The height adjusts with it.

Making an image bigger does not recover detail missing from the original.

7. Crop the area you want

Cropping removes part of the image's visible area.

1. In Photopea, choose Crop Tool.
2. Draw the rectangle to keep and confirm it. Check that you have kept important edges and labels.

8. Check compression quality

Compression reduces the data needed to store an image. Lossy compression can discard detail.

1. In Squoosh, compare the original and compressed images on either side of the divider.
2. Check text, faces and edges before downloading. The smallest result may be unsuitable.

9. Keep the project file

A project file preserves editing information that a shared image may lose.

1. Save a Photopea design as PSD if you need its layers.
2. Save a draw.io diagram as .drawio if you want to edit its shapes again.

Keep a separate PNG or other requested export for sharing.

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draw.io save dialog showing demo-flow.drawio, XML File and Download as the destination

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10. Separate an upload from a download

An upload sends data to a service. A download brings a file to your device.

1. Before selecting a file, check whether the tool processes it locally or sends it to a server.
2. After downloading, open the saved file and inspect it.

Squoosh's documentation says image processing is local. It also describes analytics, so local processing does not mean the site collects no information.

If a renamed file will not open

Restore its original name if you know it, then reopen it in the original app and export the required format. Keep the original until the exported copy works.

Sources: Microsoft file extensions (opens in a new tab); Adobe on resolution (opens in a new tab); MDN image formats (opens in a new tab); Photopea's image size (opens in a new tab) and file formats (opens in a new tab); Squoosh (opens in a new tab); draw.io formats (opens in a new tab).