When looking for an online game to join on Borderlands (I have the PS3 version), sometimes there are little icons or symbols in the information for a particular game, under the words "Plot Mission :" and the name of the mission. For example, one looks like a little box with a parachute. (I can't make out what the rest are.) What do these symbols indicate?
I am going to assume you mean these:
That is the DLC indicator. The symbols are General Knoxx, Moxxi's Underdome, and Island of Dr. Ned, from left to right. If there is a checkmark by it like so, it means that the host has the DLC.
If you see a black X in a red circle next to the image, like this,
then the host does not have that particular piece of DLC. You can see in this particular example that the host has none of the DLC at all.