Usually a small flip page thing that sits on your desk. It holds dates and has the breakdown of months on either side (of the current month).
Very useful for scrawling phone messages and assorted notes or meeting times on.
At the bottom of every flip page is usually a quote of somekind. For example, today mine reads:
"when people tell you how young you look ,they are also telling you how old you are. " - Cary Grant.
Desk calenders also come in a number of different types.. there is a Dilbert one, a Robot Man one, and pretty much any comic strip you can think of. These different types also have a daily comic strip attached.
Desk Calendars are very useful and their role in your day to day life should not be underestimated.