Ok, so I've seen these things used in threads for some time now and I have no idea what they mean. Can someone please tell me?

XD

TL/DR


What do they mean?

XD is a face, a laughing face, where the eyes are closed from heavy laughter. The D is the mouth, the X are slanted closed eyes.

TL:DR means "Too long; didn't read". People post it under forum post that are so long that you didn't even bother to read them.

TL:DR is a way of attracting a moderator's attention.

For the future, http://www.urbandictionary.com/ is great for looking things like this up.