1. It has a strictly defined number of points - precisely 12, 13 or 14 High Card Points - no more, no less.
2. It is balanced. 
   This means that the hand has no 'voids', no 'singletons', and at most one 'doubleton'.
   There are only three hand patterns that fit this definition.  The suits could be in any order:
   4-3-3-3,  4-4-3-2,  5-3-3-2

If you are in a position to open the bidding, and your hand satisfies these conditions you should always open 1NT.  (The one exception is when you have a good 5-card major suit.  Sometimes, then, it is better to open the major suit).

When you do open 1NT, you describe your hand to your partner in a single bid.  Partner rarely has to ask for more information about your hand, and can usually make a simple decision about which is the best contract to declare.

If you hold a balanced hand

When both you and your partner have balanced hands, be satisfied to play in a NT contract - don't bother bidding any suit!

Remember that you need 25 HCP to consider playing in a game contract.

1. Count your points, and add that to the 12-14 HCP that you know partner has.
2. If this definitely comes to at least 25 (i.e. if you have at least 13), you know that you have enough for game, so you should bid it - 3NT!
3. If this definitely comes to less than 25 (i.e. if you have no more than 10), then Pass - 1NT is quite high enough!
4. If you are still not sure (i.e. you have 11 or 12 points, so you might or might not have enough for game) - Bid 2NT
   This is an 'invitational' bid, also known as a limit raise.  Partner should 'accept' the invitation if he is good for his bid (i.e. if he has 14 points, or a good-looking 13).  If he has 12 points (or a poor 13), he should Pass - he knows that we don't have the 25 HCP required.

In summary, when partner opens 1NT:

• 0 - 10 HCP   Pass
• 11-12 HCP   Bid 2NT
• 13 + HCP     Bid 3NT

If you hold an unbalanced hand

If you hold a 6-card suit, you know that you have an 8-card fit, because partner's opening guarantees at least 2 cards in every suit!  If it is a major suit, you definitely want to play with that suit as trumps.  If it is a minor suit, you might prefer to play in No Trumps instead.
If you also have 13 or more HCP, you know that you have enough points for game.  In fact, if your hand is shapely, you can probably bid a game with a little less than 13 HCP.
If you know that you have enough points for game, and you know which is the right game, just bid the right game!

If you have enough points, but only a 5-card major suit, you can't be sure which is the right game.  A bid of 3♥ or 3♠ shows a hand with enough points for game, and precisely a 5-card suit.  Opener is required to bid 4 of the suit if he holds at least 3 cards in the suit, but 3NT if he has only 2 cards.  Opener must not Pass!!

If you do not have enough points for game (i.e. up to about 10 HCP), you could just pass 1NT, but if you have a 5-card suit, it will probably be safer to play in the suit instead.  A response of 2 of a suit is a simple message to partner that 'I don't have many points, and I want to play with this suit as trumps'.  This is called a 'Weak Take-Out'.  It is a decision, not a suggestion, and opener is expected to Pass!!

In summary, when partner opens 1NT, and you have an unbalanced hand:

• 0 - 10 HCP:  Bid your long suit at the 2-level.  Opener must pass now.
• 11+ HCP & a 5-card major:  Bid suit at the 3-level.  Opener must not pass.
• 11+ HCP & a 6-card major:  Bid suit at the 4-level.  There's no point bidding anything else!
• 11+ HCP & a long minor:  It's usually best to play in No Trumps, so pretend your hand is balanced, and bid accordingly.