What's different in Advent?

Advent is the three-to-four-week season before Christmas. It is a time when we prepare ourselves (spiritually and otherwise) for the celebration of the memorial of Christ's first coming and his second coming at the end of time.
       Liturgically, Advent is marked in several ways:

1. The Gloria is not sung. The reason for this is that the angels sang Gloria in excelsis Deo! on that first Christmas morning and so we refrain from singing it during our season of preparation so that it is new and fresh for us when we sing it again this Christmas.
2. Liturgical music in general should be marked by some moderation from the Ordinary Time that preceded Advent and the Christmas season to come. It need not be as spare as during Lent, but the average Catholic should notice that something is different about the music.
3. Along the same lines, parishes may do other things to mark the difference in the season. For example, one parish where I have served uses a dimmer light setting during Advent so that it is noticeable when the lights come up full on Christmas (reflecting the notion that Christ, the Light of the World has come). 

All of this falls under the principle of progressive solemnity, which is to say that Christmas is a season that should have greater solemnity than Advent, and so we do things to mark our progress on this journey from preparation to high feast.