The capital ships of a navy are its "important" warships; the ones with the heaviest firepower and armor. There is usually no formal criterion for the classification, but it is a useful concept when thinking about strategy, for instance to compare relative naval strengths in a theater of operations without having to get bogged down in the details of tonnage and gun diameters. A capital ship is generally a leading or a primary ship in a fleet.
The definition of "capital ship" was formalized in the limitation treaties of the 1920s and 30s; see Washington Naval Treaty, London Naval Treaty, and Second London Naval Treaty.
Before the advent of the all-steel navy in the late 19th century, a capital ship was a warship of the First, Second or Third rates:
1st Rate: 100 or more guns, typically carried on three or four decks. Four-deckers tended to have problems with the waterline and the lowest deck could seldom fire except on the calmest of seas.
2nd Rate: 90-98 guns
3rd Rate: 64 to 80 guns (although 64-gun third-raters were very small and not very numerous in any era).
Frigates were ships of the fourth or fifth rate; a corvette was a ship of the sixth rate.