The Ghestem convention is a set of three overcalls, to be made when the opponents open the auction with 1 (one in a random suit).
The purpose of the convention is to tell your partner your hand contains two 5-suiters and also tell him in which suits you have those 5-cards.
The agreements are very simple:

Ghestem overcalls on weak hands with two 5-suiters
  2   the highest and the lowest remaining suits
2 NT the two lowest remaining suits
3 the two highest remaining suits

You should always be prepared to recognize a Ghestem bid from your partner when the opponents open the auction. If the both the second and third hand pass, the 4th hand player still can apply this convention.
It only allowed to bid Ghestem with weak hands (6-12 points).
Ghestem denies strength"
The vulnerability must be taken into account, in order not to run the risk of losing too many points.
With stronger hands an "Take Out double" must be done. This shows strength and also promises two suits.

If you want to see examples of bridgames in which the matter, treated above, is practiced you should click on examples and choose for the appropriate convention or for any other typical call.