1) Unpassed responder's 2/1 to opener's major shows 12+ points, almost always a 5+ card suit, and is forcing to game or the 4 level of a minor.
2) 1♦—2♣ is about 99% forcing to game, or 4 level of a minor. The jump shift 1♦—3♣ is used to show an invitational hand (10-12) with 5+ clubs.
3)1♣/1♦ —2NT =10-12hcp (12-15 in SAYC)
1♣/1♦ —3NT =13-15hcp (16-17 in Sayc)
4) 1♥/1♠ — 1NT by an unpassed hand can be either 6-9 hcp (80% of the time) OR 9-12 hcp (20% of the time). This 1NT response is 100% forcing, so opener MUST bid again. Because opener needs 6+ cards in his major to rebid it after a 1-over-1, he will often have to rebid a 3-card minor in this situation. Occasionally, he will have to rebid a 2-card club suit (with 4522 and too weak to reverse).
Responder will reveal which range of 1NT she had (6-9 hcp minus, or 9+ to 12 hcp) on her rebid.
Occasionally, very occasionally, 1NT may be made with more than 12 hcp, when the hand looks hard to bid any other way, so as a defender you need to know that possibility. (E.g, if they are not using 1♥/1♠—3NT to show 12-15 and 3-card trump support, they might use 1♥/1♠—1NT and jump to game on the next round, so be wary.)
5) If the opponents interfere with bids before responder gets a chance to bid (say with overcall or t.o. dbl), the 2/1 and 1NT forcing rules are OFF, and they will bid more or less like SAYC then in competition.
6) If responder is a passed hand, bidding is also more or less like Sayc.
7) A number of conventions are standard with 2/1 and they might forget to alert them, so its handy to know which are standard:
¥Jacoby 2NT for major suit support
¥Jordan/Truscott 2NT after opps t.o. double showing limit raise or better for opener
¥New Minor Forcing (occasionally Checkback/Crowhurst)
¥Texas transfers at the 4-level
¥Lebensohl (after opponents interfere with opener's 1NT and after partner doubles a weak 2 bid)
•Cappelletti, DONT or Brozel as defenses to opponent's 1NT openers
¥Texas transfers at the 4-level
¥Inverted minors (probably OFF in competition, although the card is ambiguous)
¥Puppet Stayman after 2NT opening to allow opener to open 2NT with a 5-card major
¥Neg and responsive dbls to 3♠
¥Support dbls and redbls to 2♠
¥4-suit transfers, using 2♠ for clubs and either 2NT or 3♣ for diamonds
¥Weak jump shifts in and out of competition showing a 6 card suit and 2-5 hcp non vul, and 4-7 hcp vul. (Alternately, a few play strong Soloway jump shifts.)
¥Opener's reverse requires reversing strength (16+, some like 17+) except after 1♦—2♣ when a reverse can be made with a minimum hand.
¥Help suit game tries, and maximal overcall doubles when opps make it impossible to make a help suit game try
8) There are also a number of specialised bids which it helps to know. The strangest one is
1♥ — 1NT
Here, 2♠ is a strong raise for opener's minor, often asking opener to bid 2 or 3NT with a spade stopper. This bid is stronger than raising the minor directly, which you can do with 4-card support and 10-11 hcp. (Note that you cannot reverse the hearts and spades in this sequence.)
There are some other "trick" bids, like a 2NT rebid showing 12-14 hcp OR 18-19 hcp (example of a dual-range bid, like the 1NT response discussed above)
The concept of "fast arrival" is used. To oversimplify it, when a 2/1 bidder jumps to game immediately she is saying that she doesn't have outside controls in the unbid suits and so the jump to game is weaker than just raising partner.
Opener's jump shift rebid is not the old fashioned 19+, but rather a specialised bid showing two pretty good 5-card suits.
Opener needs a 6+ card major to rebid it after a 1/1, but only 5 cards after a 2/1. This is important to remember when defending against a 2/1 pair.