What each format is actually for, what it can and cannot hold, where it opens, and the converter to reach for when it does not. Grouped by kind, with a one-line answer to "which one should I use" at the top of each group.
Which one? Photo: JPG, or WebP for a website. Screenshot, logo, transparency: PNG. Logo that has to scale: SVG. Animation: GIF, or WebP where it is only for the web. Icon: ICO.
.heicWhat iPhones shoot: half the size of JPG, opens on Apple devices, needs converting for almost everything else.
.webpImages on websites: 25 to 35 percent smaller than JPG or PNG, and every browser has read it since 2020.
.pngScreenshots, logos, icons, anything with sharp edges or transparency. Exact pixels, big files for photos.
.jpgPhotos. Small files that open on every device ever made; no transparency, and each re-save costs a little.
.avifThe smallest photos on the web. Browsers yes, desktop programs and upload forms often no.
.gifShort looping animations. Only 256 colors, but it plays in every chat app, forum, and email client.
.svgShapes and curves instead of pixels: sharp at any size, editable as text, the right answer for a logo.
.bmpWindows' original bitmap: every pixel stored in full, no compression, enormous files.
.icoFavicons and Windows icons: the same picture at several sizes in one file.
Which one? Anything that has to print or look identical everywhere: PDF.
.pdfDocuments that look the same on every screen and printer: text, images, and layout fixed on the page.
Which one? Sharing: MP3. Editing: WAV. Keeping a lossless copy: FLAC. Came off an iPhone: M4A, convert if it will not play.
.mp3Music and podcasts. Small files that play on everything made since the nineties.
.wavUncompressed audio for recording, editing, and mastering. Perfect quality, huge files.
.flacLossless music at about half the size of WAV. Archives and audiophile libraries.
.m4aAAC audio, the Apple-flavored MP3: better sound per megabyte, default on iPhones and iTunes.
.oggOpen audio for games and streaming: Vorbis for music, Opus for voice. Not on Apple by default.
Which one? Sharing or uploading: MP4. Came off an iPhone: MOV, convert to MP4 if it will not play. Web page: WebM. Keeping everything, subtitles included: MKV.
.mp4The container that plays everywhere: phones, browsers, TVs, Discord, email attachments.
.movWhat iPhones and Macs record. Same codecs as MP4 inside, fewer players outside Apple.
.mkvHolds any codec, several audio tracks, subtitles, and chapters. VLC loves it; phones and TVs often refuse it.
.webmOpen web video from Google: browsers play it natively, Apple devices only sometimes.
.aviMicrosoft's 1992 container. Still readable everywhere, but what is inside it is usually ancient.
Which one? Handing the file to someone, or uploading it: SRT. Video on your own web page: VTT. Styling, positioning, or karaoke: ASS.
.srtThe plain-text subtitle file everything accepts: a number, a timecode, the line, a blank line.
.vttThe subtitle format HTML video requires: SRT with a header, periods in the timecodes, and optional styling.
.assAdvanced SubStation Alpha: styled, positioned, karaoke-timed subtitles. What fansubs are made of.
Which one? Shipping in a game: DDS with mipmaps. Source art for an engine or a mod: TGA or PNG.
.ddsGame textures: compressed in a form the graphics card reads directly, with mipmaps built in.
.tgaSimple lossless textures with a clean alpha channel; engines and modding tools accept it, browsers do not.