Wikipedia
Bachelor party
A bachelor party (United States, Canada, South Africa), stag party, stag night, or stag do (Canada, UK, Ireland, and New Zealand), bulls party (South Africa), or bucks party or bucks 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 grooms 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.
Variations
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 person entering into wedlock.
Some also chose instead to hold a so-called Stag and Doe party in the US or a hag party or hag do in the UK ("hag" being a combination of the words "hen" and "stag"), in which both the bride and groom attend.
Australia
Bucks nights may feature a stripper, topless waitresses and/or poker games. The grooms mates often, after heavy drinking, subject the groom to various humiliations, some read more at » http://en.wikipedia.org/wiki/Bachelor+party