This is a derp face meme. It is the first, and possibly the most popular, meme on the internet. It symbolizes being silly or sometimes stupid and ignorant.
This is the ever popular meme, the lol face. It is used when you are laughing really hard, or find something amusing.
This is the funniest of all of the memes. It means that a person made someone else look or do something stupid, and it caused the receiver to be mad.
The final most popular meme face is the challenge accepted face. It means that someone told you to do something difficult and you have decided that you will do it.