Gatsby's parties are large, flamboyant, over-the-top affairs which cater to the guests' every whim. Although formal invitations are sent, the parties are attended by everyone who wishes to have a good time. As such, the parties are attended by a cornucopia of guests, from those with "old" money from East Egg, to those with "new" money from West Egg and those with no money who wish to create the impression that they have money or are on the up-and-up.
Evidently, some of Gatsby's more sinister companions also attend these parties as well as characters such as Klipspringer, who is a parasite. Anyway, most of the attendees are self-indulgent and self-absorbed characters who have no whim other than to exploit and abuse Gatsby's generosity at no cost to themselves. F. Scott Fitzgerald satirizes the empty and shallow self-indulgence commonly displayed during the so-called "Jazz-Age."
The parties are obviously enormously public and well-attended occasions and the raucous and ill-disciplined behavior of many of the guests is well-documented in the novel. The purpose of these parties is an attempt by Jay Gatsby to draw Daisy closer. He hopes that the parties will pique her interest and that she will attend so that he can rekindle their relationship. It is his ideal that they should be together again.
Tom Buchanan's parties in his New York apartment differ from Gatsby's in that they are private little affairs between Tom and a select number of guests, mostly companions related to Myrtle in some way or another. The actual purpose is for Tom to indulge his affair with Myrtle, a married woman. It soothes his ego that he can openly have this affair without being concerned about any complications which may arise, for his money would take care of any difficulty in this regard.
In both instances, the guests freely indulge in alcohol and let go of any inhibitions they otherwise might have. Nick Carraway, for example, gets completely drunk at one of Tom's get-togethers. Tom also displays his true colors and affections during one of these occasions - firstly, when he breaks Myrtle's nose when she derisively calls out Daisy's name and secondly, when he is involved in a confrontation with Gatsby about his affair with Daisy and it is here that he makes known his suspicions about Jay's criminal activities. The guests at both parties are similar in the sense that they are parasites feeding off the generosity of their hosts.
The parties at both Tom and Gatsby's are both sodden affairs, filled with too much alcohol and it effects. However, the motivation behind the parties is different. Gatsby has parties simply to attract the attention of Daisy Buchanan.No one seems to get hurt at the parties and everyone seems to have a good time. But Gatsby himself doesn't even attend his own parties and is rarely seen during them. Tom wants to have a good time with his mistress, Myrtle. He attends the parties and in fact, becomes on one the biggest boors there. At one memorable party, he gets into an argument with Myrtle and hits her. No one is impressed and most people end up embarrassed at their hosts, unlike Gatsby's parties, where most people never see their host.