Well... There are different examples of what'd be considered as spam.
Usually, it's a thread advertising the sale of products that have nothing to do with the ambiance of the thread. Say for example, someone posts a thread titled "Blackberry Telephones for Sale, Cheap!" in the B/S/T section. That has nothing to do with the Muppets or any of their productions, so it gets reported as spam.
Other times, it's some person who's logged on and registered as a new member who posts super short messages or messages that border on the lude crude and rude side of the spectrum. Those posts are spotted from time to time and get reported as well.
Check the FAQ if you have any further queries and I hope this helped explain things a little.