File types, explained

CSS file

.css · text/css

A CSS file is a plain-text stylesheet that tells a browser how to display a web page: colours, fonts, spacing and layout. This guide explains what's inside one, how browsers decide which rule wins, the quirks worth knowing, and has free sample CSS files to download for testing.

HTML file

.html, .htm · text/html

An HTML file is a plain-text document that a web browser turns into a web page. This guide explains what's inside one, how browsers read it, the quirks worth knowing, and has free sample HTML files to download for testing.

JavaScript file

.js, .mjs · text/javascript

A JavaScript file is a plain-text program that runs in a web browser, or on a server with Node.js. This guide explains what's inside one, the difference between scripts and modules, the errors worth knowing, and has free sample JavaScript files to download for testing.

JSON file

.json · application/json

A JSON file is a plain-text file that stores structured data as objects, lists, text and numbers, in a form both people and programs can read. This guide explains the syntax, the mistakes that stop a file from parsing, the quirks worth knowing, and has free sample JSON files to download for testing.

Markdown file

.md · text/markdown

A Markdown file is a plain-text document that uses simple symbols, like # for a heading and ** for bold, which a renderer turns into formatted text. This guide explains what's inside one, how it becomes HTML, the quirks of its many flavours, and has free sample Markdown files to download for testing.

TXT file

.txt · text/plain

A TXT file is a plain-text file: characters and line breaks, with no formatting at all. This guide explains what's inside one, how encodings and line endings work, the quirks worth knowing, and has free sample TXT files to download for testing.

XML file

.xml · application/xml

An XML file is a plain-text file that stores data and documents inside tags you define yourself, following strict rules that any XML parser can check. This guide explains what's inside one, how parsers read it, the formats built on it and its security pitfalls, and has free sample XML files to download for testing.

DOCX file

.docx · application/vnd.openxmlformats-officedocument.wordprocessingml.document

A DOCX file is a Microsoft Word document: a ZIP package of XML files holding the text, formatting, images and settings. This guide explains what's inside one, how it differs from DOC and PDF, the quirks worth knowing, and has free sample DOCX files to download for testing.

PDF file

.pdf · application/pdf

A PDF file is a document that looks the same on every screen and printer, with its fonts, images and layout fixed in place. This guide explains what's inside one, how readers open it, the quirks worth knowing, and has free sample PDF files to download for testing.

CSV file

.csv · text/csv

A CSV file is a plain-text table: one record per line, with the fields separated by commas. This guide explains the quoting rules, why spreadsheets so often change what's inside, how CSV compares with XLSX and JSON, and has free sample CSV files to download for testing.

XLSX file

.xlsx · application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

An XLSX file is an Excel workbook: a ZIP archive of XML files that holds sheets, formulas, formatting and charts. This guide explains what's inside one, how it stores numbers and dates, how it differs from XLS, XLSM and CSV, and has free sample XLSX files to download for testing.

GIF file

.gif · image/gif

A GIF file is an image with at most 256 colours per frame that can hold several frames, which is why it became the web's format for short looping animations. This guide explains what's inside one, how animation and transparency work, and has free sample GIF files to download for testing.

ICO file

.ico · image/vnd.microsoft.icon

An ICO file is a small container that holds the same icon at several sizes, used for Windows program icons and website favicons. This guide explains what's inside one, how apps pick a size, the favicon quirks worth knowing, and has free sample ICO files to download for testing.

JPEG file

.jpg, .jpeg, .jpe · image/jpeg

A JPEG file is a compressed photo: it throws away detail your eye is unlikely to miss, so pictures take a fraction of the space. This guide explains what's inside one, how the compression works, the quirks worth knowing, and has free sample JPG files to download for testing.

PNG file

.png · image/png

A PNG file is a lossless image: every pixel comes back exactly as it was saved, transparency included. This guide explains what's inside one, when PNG is the right choice and when it isn't, and has free sample PNG files to download for testing.

SVG file

.svg · image/svg+xml

An SVG file is a text file that describes an image as shapes, so it stays sharp at any size. This guide explains what's inside one, how browsers draw it, the quirks and security issues worth knowing, and has free sample SVG files to download for testing.

WebP file

.webp · image/webp

A WebP file is an image format from Google that can be lossy like JPEG or lossless like PNG, with transparency and animation, usually in a smaller file. This guide explains what's inside one, how it compares with the formats it replaces, and has free sample WebP files to download for testing.

MP4 file

.mp4 · video/mp4

An MP4 file is a container that holds video, audio and the information a player needs to keep them in sync. This guide explains what's inside one, why the same .mp4 plays on one device and not another, the quirks worth knowing, and has free sample MP4 files to download for testing.

MP3 file

.mp3 · audio/mpeg

An MP3 file is compressed audio that leaves out sounds you're unlikely to hear, making it about a tenth the size of a CD-quality recording. This guide explains what's inside one, how tags and bit rates work, the quirks worth knowing, and has free sample MP3 files to download for testing.

WAV file

.wav · audio/wav

A WAV file is a recording stored as raw, usually uncompressed samples, the format audio software works with before anything is compressed. This guide explains what's inside one, how its chunks fit together, the quirks worth knowing, and has free sample WAV files to download for testing.

ZIP file

.zip · application/zip

A ZIP file is an archive: many files and folders packed into one, each usually compressed. This guide explains what's inside one, how its central directory works, the security traps worth knowing, and has free sample ZIP files to download for testing.