The following is an excerpt from Wikipedia's article on "Bachelor Party"

A bachelor party, stag party, stag night, or stag do (UK, Ireland, and New Zealand), bull's party (South Africa), or buck's party or buck's night (Australia) is a party held for a bachelor shortly before he enters marriage, to make the most of his final opportunity to engage in activities a new partner might not approve of, or merely to spend time with his male friends (who are often in his wedding party afterwards).

Events

A bachelor party may involve activities beyond the usual party and social gathering ingredients (often drinking alcohol and gambling), such as going to a strip club or hiring a stripper, and in some traditions more hazing-like tests and pranks at the future groom's expense, which shows the whole thing is also a rite of passage from bachelorhood (associated with an adolescent lifestyle, often in the common past of most participants, e.g. in their student years) to "more responsible" marital life.

The task of organizing a bachelor party is often traditionally assigned to a male sibling of the bachelor or to the best man. Otherwise, any (close and/or reputedly party-minded) male friend will organize it. The planned activities of a bachelor party are traditionally kept secret from the groom.

It has also become common for a similar party to be held for the bride-to-be. This is known as a bachelorette party or Hen party. Formally, a party in honor of the bride-to-be is hosted by others. The expenditure, however, needs to be carried out by the the person entering into wedlock.

A tame version is the so-called Stag and Doe party, in which both the bride and groom attend.

Bachelor parties in the US often entail the mass consumption of alcohol, the hiring of a stripper and general rowdiness toward which the bride might not have a positive reaction (especially since the bride is not typically among the invitees). Increasingly, bachelor parties have come to symbolize the last time when the groom is free of the influence of his new wife.