What is a .ico file?

You need a favicon for your website or an icon for a Windows application. That means you need an ICO file — the format that packs multiple image sizes (16×16, 32×32, 48×48, 256×256) and colour depths into a single file so the operating system can pick the right version for each context.

ICO is the original favicon format — that little icon in your browser tab. Modern websites have mostly moved to PNG or SVG favicons, but ICO remains the only format guaranteed to work in every browser, including ancient versions of Internet Explorer. For Windows applications, ICO is still required — it's what the taskbar, desktop shortcuts, and file explorer use. The format also supports transparency, though older ICO tools sometimes handle it poorly.

GIMP (free) creates and edits ICO files. IcoFX ($60, Windows) is a dedicated icon editor. For web favicons, fwip can convert a PNG to ICO. Online favicon generators handle the multi-size packing automatically. The simplest modern approach for websites is an SVG favicon with an ICO fallback — you get scalable quality for modern browsers and compatibility for legacy ones.