A "cuebid" is a bid of the opponent's suit.

This is not the same as a "control" bid as in this auction:

West	North	East	South
1	Pass	3	Pass
4

Here 4 shows the ace or king (or a singleton or void) in diamonds and slam interest.

We should use the term "cuebid" only when bidding a suit which the opponents have bid.

These cuebids come in many flavors, the main ones being:

Michaels Cuebid

This popular convention uses the following scheme:

West	North
1	2	MAJORS
1	2	MAJORS
1	2	+minor
1	2	+minor

There is more on Michaels bids by clicking on it.

Cuebid after our takeout double

West	North	East	South
1	Double	Pass	2

South can't want to play in clubs (he could have passed the double). This bid shows a good hand with no clear direction. Partnerships need to discuss how high this bid is forcing.

Western cuebid

This term causes a lot of confusion. To read more about this so-called "Western" cuebid, click the link.