Structurally, the above argument(Circular Reasoning) would look something like this:

* x implies y
* Assume x
* Therefore, y

Make a little more sense? Good. Let's move on to a slightly more complex version of this type of argument:

* Barack Obama will beat Hillary Clinton in the California democratic primary this coming Tuesday provided young people like me just get off their lazy asses and vote!
* It was?
* Fuck.

While circular arguments are technically valid, they aren't really recommended unless you are trying to prove the theory of intelligent design or something comparable. Again, I'm no logician, but it's a basic explanation that I hope will suffice.