The Great Hall, where the students come to have their meals. This is also where they come straight after they have travelled on the Hogwarts Express for the Beginning of year feast. It is also when the First-Years get sorted into their houses by the Famous Sorting Hat.
The other major feasts include the Halloween Feast and the End of Year Feast, where the House Cup is awarded to the House with the Highest about of house points that have been awarded throughout the year. (There will be a point system up soon, but no just yet).
There are four long tables for the Four Houses. This is the order as you come on in through the large doors, Slytherin on your far left, Hufflepuff, then Ravenclaw, with Gryffindor on the far right. Up head is where the High Table, where the teachers sit, with the Headmaster or Headmistress sitting dead in the middle and the Deputy Headmaster or Headmistress sits on the Head's right.
There are also two doors near the High Table (which is on a slight dais), the door on the left is where the first years come out of when they are to be presented to the school and sorted into their houses. The door on the right, many believe it is simply a sort of short cut for teachers and other students think there is more than just the one door up there.
While during the major feasts students are only allowed to sit at their house tables, but during other times - i.e. every day meals and study hall - students are allowed to sit at the other house tables to be with their friends.
The Great Hall is also where the OWL examinations are held.