A smurf is someone who intentionally will de-rank themselves down to a much lower rank than their actual rank. The reason they do this is so that way they can beat opponents easily and for some odd reason, it's really gratifying for them. It's pretty easy to spot a smurf, they will be doing a lot more advanced things in-game and along with that, they will be playing at a much faster pace. The reasons that they are not fun to play with is because you are going to lose, you think you're playing someone close to your rank, and it's really frustrating. Although it sucks playing against a smurf you can gain a lot more than you can lose. Take the opportunity to learn why they are better than you, watch for your own mistakes and see how the smurf takes advantage from those mistakes. If you can do this you will be able to rank up much faster and won't have to deal with smurfs in higher ranks.