Tech
What Is a File Format? Types and Uses Explained
What is a file format? Extensions, containers vs codecs, lossy vs lossless, and the right format for documents, images, video, audio and archives.
· 5 min read · RunConvert Team
Every file on your computer is just a long string of ones and zeros. A file format is the agreement that tells software how to read those bits. It decides whether a given sequence becomes a photo, a song, a spreadsheet or a page of text. When an app "can't open" a file, it almost always means the app doesn't understand that file's format.
This guide explains file formats in plain English. It covers what the extension at the end of a filename really means, the difference between containers and codecs, lossy versus lossless compression, and which formats to use for everyday files.
Extensions Are Labels, Not the Format Itself
The letters after the dot, such as .pdf, .jpg and .mp4, are the file extension. They're a hint to your operating system about which app should open the file. The real format is defined by the data inside.
That's why renaming photo.heic to photo.jpg doesn't convert anything: the data is still HEIC, and most apps will refuse it or show an error. To actually change the format, the data has to be rewritten, which is what a converter does. (For HEIC specifically, see how to convert HEIC to JPG on any device.)
Open vs Proprietary Formats
- Open formats have published specifications anyone can implement: PDF, PNG, ODT, FLAC, WebM, EPUB. They tend to stay readable for decades.
- Proprietary formats are controlled by one company, though many are documented and widely supported. Older Microsoft Office formats like DOC and XLS, and several RAW camera formats, are examples.
For anything you want to keep long-term, open and widely supported formats are the safer bet.
Lossy vs Lossless Compression
Most formats compress data to save space, in one of two ways:
- Lossless compression shrinks the file and restores every bit exactly when opened. Examples: PNG, FLAC, ZIP, WAV (uncompressed), and lossless WebP.
- Lossy compression throws away detail it predicts you won't notice, for much smaller files. Examples: JPG, MP3, AAC, H.264 video, and lossy WebP.
Two practical rules follow:
- Converting lossy to lossless (JPG → PNG) doesn't restore lost detail. It only stops further loss.
- Converting lossy to lossy repeatedly (MP3 → AAC → MP3) compounds the loss. Always convert from the original.
Containers vs Codecs (Video and Audio)
Video and audio formats have two layers, which is why "it's an MP4 but it won't play" happens:
- The container is the box: MP4, MOV, MKV, WebM, AVI. It holds the video, audio, subtitles and metadata together.
- The codec is how each stream inside is compressed: H.264, H.265/HEVC, VP9 and AV1 for video; AAC, MP3, Opus and FLAC for audio.
An MP4 holding H.264 plays almost anywhere. An MP4 holding H.265 may not play on an older Windows PC. Same box, different contents. Our MOV vs MP4 article shows how this plays out with iPhone videos.
The Main Types of File Format
Documents
| Format | What it's for |
|---|---|
| Fixed layout that looks identical everywhere. Best for sharing and printing | |
| DOCX | Editable Word documents |
| ODT | LibreOffice's open document format |
| RTF / TXT | Simple, universally readable text |
Need to edit a PDF? See how to convert PDF to Word without losing formatting.
Images
| Format | Strength |
|---|---|
| JPG | Photos, small files, universal |
| PNG | Lossless, transparency, sharp text and graphics |
| WebP | Modern web format, smaller than JPG and PNG. See WebP to PNG |
| HEIC | iPhone default, very efficient, limited support outside Apple |
| GIF | Simple animation, 256 colours |
| SVG | Vector graphics that scale to any size |
| RAW (CR2, NEF, ARW…) | Unprocessed camera sensor data for editing |
Video
MP4 is the universal choice. MOV is Apple's, MKV holds multiple tracks and subtitles (and is easy to convert to MP4), and WebM is built for the web. Our online video converter guide explains which to pick.
Audio
MP3 is universal, AAC/M4A is efficient and Apple-friendly, WAV is uncompressed, and FLAC is lossless and smaller than WAV. The MP3 vs WAV vs FLAC comparison goes deeper.
Spreadsheets and presentations
XLSX and PPTX are the Microsoft standards, ODS and ODP their open equivalents, and CSV is plain-text tabular data that every database and spreadsheet understands.
Ebooks
EPUB is the open standard used by most e-readers and apps; MOBI and AZW3 are Kindle formats.
Archives
ZIP opens everywhere, 7Z compresses tighter, RAR is common for downloads, and TAR.GZ is standard on Linux. Stuck with a RAR? See how to open a RAR file.
Why Convert at All?
You convert a file when the format you have doesn't suit where it needs to go:
- Compatibility: an app, device or website only accepts certain formats.
- Size: a more efficient format or setting makes it small enough to send.
- Editing: turning a fixed format like PDF into an editable one like DOCX.
- Longevity: moving from an obscure or proprietary format to an open, widely supported one.
The One-Line Summary
A file format is the set of rules that turns bits into something useful, and the extension is just its label. When a file won't open or is too big, converting it to a better-supported format usually fixes the problem. You can do that free with the RunConvert converter, which covers more than 3,000 conversions. For the bigger picture, read our complete guide to file conversion.
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