Unusual NT is a convention which overlaps with Michael's.
2NT is used as an overcall over a major suit. It shows at least five cards in both minor suits.
This bid can be used after both opponents have bid.
With a weak hand and little support the responder bids her longest minor or, if her minors are of equal length, 3C. This allows an escape to diamonds if clubs are doubled.
With a weak hand and 4 card support, the responder can preempt with a bid at four level.
1H - 2NT - PASS - 4C
This bid is non forcing.
With a strong hand and stops in the major suits the responder can bid 3NT, which is to play.
With a strong hand and an interest in game or slam, the responder cue bids the opponent's suit.
With fewer than 10 points the overcaller bids 4C.
With more than 10 points the overcaller bids 4D.
The responder then decides the final contract.