Party is a term that can be defined in a number of ways, most of which relate to collective groupings.
'Party' can have several meanings, including:
A "traditional" festive party can be arranged for a variety of reasons. Most often a party is held in the event of someone's birthday, although there are other frequent occasions such as anniversaries (of practically anything), weddings, graduations, or holidays.
Housewarming parties are not unusual for an individual who has just moved into a new home; often these parties happen as a result of someone who has just moved away from the house that they grew up in.
Bachelor or Bachelorette parties are not uncommon for each member of the couple that is to be wed. These parties have a reputation for being wild and crazy as they are the apparent "last-time" the single couple will be able to enjoy themselves in such a manner.
There are other types of parties that don't explicitly have the word "party" in them. "Baby Showers" are parties that are focused on the birth of a new child and the celebration of that event with the mother-to-be. Such parties are semi-formal in both atmosphere and content as the environment is one of support and comfort.
There are even online parties, which can involve like-minded people using virtual interactions to create a festive atmosphere. One example is the Citizendium monthly Write-a-Thon party.