Where there are just rows of chairs behind each other (like a theater).
Where there are banquet tables or penny tables (long narrow banquet tables) with chairs behind each table, all facing forward (like a classroom with desks).
Generally a few tables with chairs for those who want to sit down, but this style of room set up is geared towards a standing reception. This is generally more common for cocktail and hors d'oeuvres reception.
Round tables of 8-12 people per table (depending on size of the table)
Often referred to as "horseshoe" shape, this is a bit more limiting as far as how many people can fit on the outside. Some groups choose to use the inside and outside of the U-shape. No matter what your choice, do check with your catering manager to determine what would maximize the room space with your group's size, audio visual needs, etc.
This is generally one large "square" or "rectangle" that seats up to about 30 people around the edges. This style is not usually conducive to larger groups and works better with smaller, more intimate meetings.