Penalties for undertricks
If a team doesn't make the number of tricks they have stated they will make they incur a penalty of 50 points for every trick they are short if they are not vulnerable and 100 points a trick if they are vulnerable. (In duplicate, vulnerability is pre determined and in contract it depends who already has one game).
Doubling and re-doubling
If the opponents don't think the number of tricks bid can be made by the declarer they may double the contract. This increases the penalties levied if the required number of tricks is not made. The chart above shows the penalties that are involved when non vulnerable. (Notice how the penalties become very severe for doubled and re-doubled contracts). The declarer can also re-double if they are pretty sure that they are going to make the contract. This increases the points awarded if the contract is made and makes the penalties very severe if the contract isn't made. In practice it's quite rare for a contract to be re-doubled. The chart below shows the penalties involved where the pair are vulnerable. There are also bonuses for making a doubled contract.
On each board in duplicate bridge there is a slip of paper which is a traveling score sheet and is called the "traveler". This is filled in after every hand and all the travelers are collected at the end of the tournament. The information on them is used to allocate match points which in turn indicate the overall results. Traditionally the traveler is filled in by the player sitting in the north position but this is not a rule. Some clubs dispense with the traveler completely and do the scoring electronically nowadays.