Grind size is one of the biggest levers on how coffee tastes. It controls how fast water pulls flavor out of the grounds.
Why grind matters
Smaller grounds have more surface area, so water extracts them faster. Too fine for a method over-extracts and tastes bitter; too coarse under-extracts and tastes sour and weak.
From coarse to fine
Cold brew and French press want a coarse grind for their long brew times. Drip and pour over sit in the middle. Espresso and Turkish coffee need a fine to extra-fine grind for their fast, pressurized brews.
Common mistakes
Pre-ground coffee goes stale quickly and is usually one fixed size that does not suit every brewer. A burr grinder set to match your method makes a bigger difference than most people expect.
See the Coffee Grind Size Chart and the Grind Size Comparison.