WOFF wraps a TTF or OTF in a compressed container (zlib) optimized for web download. Standardized by the W3C in 2012. Smaller than the source font but larger than WOFF2 (which uses Brotli). Supported by every browser since IE9. Many sites ship WOFF as a fallback for older browsers that don't support WOFF2.
How to open
Browsers consume WOFF via CSS `@font-face` declarations. Desktop systems don't install WOFF directly — convert to TTF first. Font editing tools (FontForge, Glyphs, FontLab) read WOFF for editing.