After an opening bid, the partnership enters into a dialogue in 
which they attempt to determine what the best contract should be.  
In most auctions, one of the two partners, at some point, makes 
a "limiting" bid.  When that happens, the other partner becomes 
the "captain" of that particular auction and more or less "takes 
control".  The first problem is having a partnership where both 
players are aware of this and don't try to dominate after making 
a limit bid.  For instance, East opens one no trump.  East is 
limited to a point range of 15 to 17 high card points with no 
singleton or void.  West is the captain of this auction.  Period.  
The only decision left to East is if West makes an invitational 
bid.  Let's say that West transfers with a two heart bid.  East 
bids two spades and West now bids two no trump.  He is inviting 
game in either no trump or spades - but if he rebids three no 
trump, he is demanding game in no trump or spades.  In the first 
situation, opener looks at how many spades he has, if only two, 
he will select no trump, two with a minimum opener or three with 
a maximum opener.  If the rebid is three no trump, opener will 
pass with two spades or bid four spades with more spades.  
That's the only option available.  

Other auctions are not so clear all the time.  A new suit by an 
unpassed hand is forcing (unless the partnership has some specific 
agreement otherwise in specific situations).  A new suit at the 
three level is always forcing.  A reverse bid is forcing if the 
responder really had his first response.  A jump shift is always 
forcing unless it is a jump shift to two no trump, which is not 
forcing.

This is why experienced partners play new minor forcing and 
fourth suit forcing.  These are both good conventions that will 
clarify difficult bidding situations.  

Let's look at some examples.

Opener bids one heart.  Responder bids one no trump.  Opener is 
captain and responder is showing, specifically, that he does not 
have three hearts or four+ spades.  He also does not have a good 
enough suit or a good enough hand to make a bid at the two level.  
If opener now bids a new suit, it is forcing.  Either to two 
hearts or three of the new suit.  If opener rebids hearts or 
raises no trump, responder has to re-evaluate and bid accordingly.

Opener bids one club.  Responder bids one no trump.  This shows 
a hand with 6 to 9 (or a bad 10) points without four diamonds, 
four hearts or four spades.  If you have five or more clubs you 
should really be raising clubs and not bidding no trump, so you 
should really only be 3-3-3-4.  Opener now bids two hearts.  This 
is a reverse and is forcing.  He knows that responder does not 
have four hearts, so he must be looking for something else, like 
a control in another suit.  A two no trump response now would 
show 6 or 7 points with nothing special to say.  Three hearts 
would be decent three car support.

Opener bids one diamond.  Responder bids two clubs (showing 5+ 
clubs and 10+ points).  Opener bids two hearts.  This is not 
a reverse!  He was forced to the two level and is just bidding 
a four card suit for the purpose of investigating a no trump 
contract.  If he had three or four clubs he should just raise 
clubs with a normal opener.

In each situation, you must analyze not only what partner has 
bid, but what he has bypassed in order to make that bid.  
Every sequence should show some very specific details that 
will help in determining the final contract.

All of this will be clearer when you have some more experience 
and have the opportunity to review the mistakes and recognize 
what went wrong.
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