According to the Urban Dictionary, a pun is “a play on words, usually used for comic reception,” that is, used to make people laugh or groan.  A friend sent me this picture on Facebook (it is not mine and I do now know where it comes from) and it is a good example of some puns:

Let me explain a bit:

A coffee cup is called a mug.  To mug someone is to assault them with with the intent to rob them.  This  is a pun because it plays with the word mug in reference to stealing a coffee cup, a mug.

Click on the picture to read more mugs (and I will continue to explain the puns in this picture this week).