File extensions (.jpg, .pdf, .mp4) tell your computer which program to use to open a file. But they also encode a lot of choices about quality, compatibility, and size. Understanding what each format is designed for saves time when something won't open and helps you choose the right format when saving or exporting files.
Image Formats
JPEG / JPG
The most widely supported image format for photos. JPEG uses lossy compression — it discards some image data to reduce file size. Every time you save a JPEG, quality degrades slightly. JPEG does not support transparency.
Use for: Photos you're sharing, emailing, or uploading to websites. Avoid for: Screenshots with text, logos, or images you'll edit repeatedly.
PNG
PNG uses lossless compression — quality never degrades when you save it. PNG supports transparency (useful for logos on different backgrounds). PNG files are larger than JPEG for photographs.
Use for: Screenshots, logos, graphics with text, images that need transparency. Avoid for: High-resolution photos where file size matters.
WebP
Google's modern image format that provides better compression than JPEG and PNG — roughly 30% smaller than JPEG at equivalent quality, with transparency support. All major browsers now support WebP. You may encounter it when downloading images from modern websites.
Use for: Web publishing. Convert to JPEG or PNG if you need to open it in older software.
HEIC
Apple's default photo format on iPhones (iOS 11+). Smaller than JPEG at the same quality but not widely supported outside Apple platforms. Windows requires a paid codec or the free HEIF Image Extensions app to open HEIC files.
GIF
An old format (1987) that supports simple animations and is limited to 256 colors. Still widely used for short looping animations on the web. For static images, PNG is strictly better.
TIFF / TIF
Lossless, high-quality format used in professional photography and printing. Files are very large. Rarely used outside professional or archival contexts.
Document Formats
PDF (Portable Document Format) preserves layout exactly regardless of device or software. What you see is what the recipient sees. It's the standard for documents intended to be read but not edited — contracts, forms, reports, books.
PDFs can be either text-based (copyable text) or image-based (a scan). Image-based PDFs can't be searched or edited without OCR (optical character recognition) software first.
DOCX
Microsoft Word's default format since Word 2007. Widely supported by Word, Google Docs, LibreOffice, and Pages. Unlike PDF, DOCX is designed for editing — it reflows text, adapts to fonts installed on the reader's machine, and may look different on different computers.
Use for: Documents you or others need to edit. Use PDF for: Final versions that should look identical everywhere.
TXT
Plain text — no formatting, no fonts, no images. Opens on every device ever made. Good for notes, code, config files, and anything that doesn't need formatting.
XLSX
Microsoft Excel's format for spreadsheets. Supported by Google Sheets, LibreOffice Calc, and Numbers. Contains cells, formulas, and (optionally) macros. .xls is the older, now-deprecated Excel format — convert to .xlsx if you encounter it.
Audio Formats
MP3
The universal audio format. Lossy compression: some audio data is discarded to reduce file size. Quality depends on the bitrate — 128 kbps is acceptable for casual listening; 320 kbps is near-indistinguishable from lossless for most people on most speakers.
AAC
Apple's preferred audio format (used in iTunes and Apple Music). Technically more efficient than MP3 at the same bitrate — sounds better at 128 kbps than MP3 does — but compatibility is slightly narrower. Supported natively on Apple devices, Android, and most modern players.
FLAC
Free Lossless Audio Codec. No quality loss — the decoded audio is bit-for-bit identical to the original. Files are roughly 50–60% the size of uncompressed WAV. The format for archiving music or listening on high-quality audio hardware. Not supported by iTunes/Apple Music natively.
WAV
Uncompressed audio — the raw data with no compression. Files are very large (a three-minute song at CD quality is ~30 MB). Used in professional audio production and video editing where lossless quality and direct manipulation matter.
Video Formats
MP4 (H.264)
The standard container format for video on the web, phones, and most software. .mp4 files typically use H.264 video compression and AAC audio. Near-universal compatibility — plays on every device without plugins.
MOV
Apple's QuickTime format. Functionally similar to MP4 (can contain the same H.264/H.265 video), but native to macOS and iOS. Windows can play .mov files with QuickTime for Windows installed or with VLC.
MKV
A flexible open-source container that can hold multiple audio tracks, subtitle streams, and chapters — widely used for rips of films and TV. Not supported by all players natively, but VLC (free, open source) handles MKV reliably on every platform.
Archive Formats
ZIP
The universal archive format. Windows, macOS, and Linux all handle ZIP natively — no software needed to extract. Good for bundling files for sharing. Compression is moderate.
7Z
7-Zip format with significantly better compression than ZIP — often 30–50% smaller for the same files. Requires 7-Zip (free, open source) on Windows or The Unarchiver on Mac to extract. The right choice when compression ratio matters more than universal compatibility.
RAR
Commonly used for files distributed online, especially multi-part archives (.part1.rar, .part2.rar). Requires WinRAR or 7-Zip to extract. 7-Zip handles RAR extraction for free.
When a File Won't Open
If you can't open a file, work through this list:
- Check the extension: Right-click, Properties (Windows) or Get Info (Mac). Is the extension what you expect?
- Install the right app: VLC opens nearly any audio/video format. 7-Zip extracts nearly any archive. LibreOffice opens most document formats.
- Verify the file isn't corrupt: If it downloaded partially, delete it and download again.
- Convert it: Use a format-specific converter (see our guides on HEIC and PDF) to get it into a format your existing software handles.