Yep, it's the penitential season before Christmas, in the same way that Lent is the penitential season before Easter. The liturgical color is violet, so you'll see priests wearing violet and various cloths, etc, that are violet. Most churches in the US have an advent wreath with 4 candles which are lit as we go through Advent. It's a time of reflection and preparation, before we bust out with Christmas and PAR-TAY!

Then we have the season (yes, SEASON, not just day) of Christmas. So while the stores are packing up the Christmas stuff they've had up since before Halloween at midnight on Christmas Eve, we're just starting our Christmas celebration, churches are decorated for Christmas, etc.
