The Jack-of-all-trades is a role with several one-shot night abilities - typically abilities centered around investigating, protecting and killing. The Jack-of-all-trades may use whichever ability they want on a specific Night, but can use each one only once.
Jack-of-all-trades is very commonly abbreviated JOAT. This role has been seen as both Town and Mafia.
The Werewolf-flavored equivalent to JOAT is Witch.
"Jack-of-all-trades" is a catch-all term for any role that has multiple one-shot abilities, so the variations are as extensive as a given moderator's imagination. In Normal games, JOATs may have any combination of whitelisted abilities.
The Jack-of-all-trades shares many similarities with an Inventor.
Use and Power
A Jack-of-all-trades is somewhat useful in setups because it produces less swing than other power roles, because each of its powers is one-shot, which puts a definite limit as to the number of investigations, protections, etc, that the Jack-of-all-trades will be able to perform. However, it also concentrates the effects of many power roles into the hands of one player, who may die early or be ineffective with power roles in general, so it balances out. Its variety is rather appealing in setups, though.
The power of a JOAT is a function of how strong its individual abilities are.