Impossibru is when you find something out something or see something that looks impossible, or seems like it is impossible.
Philosoraptor is a raptor that asks very deep, but silly, questions. He is used for asking someone something amusing, but thought provoking.
This is the grumpy cat. He is used when you or someone else clearly does not find something amusing.
This meme signifies that you have lost, gotten owned, or just given up. It is used commonly in arguments.