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match is the heart of F#. It's a switch that destructures by shape:

let describe n =
    match n with
    | 0 -> "zero"
    | n when n > 0 -> sprintf "positive: %d" n
    | n -> sprintf "negative: %d" n      // catches everything else

Patterns:

• 0 — literal
• n — bind to name
• n when ... — guard
• _ — wildcard (matches anything, doesn't bind)
• (a, b) — tuple
• [x; y; z] — list of exact length 3
• head :: tail — non-empty list (head is first, tail is the rest)
• [] — empty list
• Some x / None — option destructuring

Matches must be exhaustive. The compiler warns if you miss a case. This is huge for safety — when you add a new variant, the compiler tells you exactly which match expressions need updating.