What Happens At A 'Contested' Convention?

Folks are starting to talk more and more about the possibility of a "brokered" or "contested" Republican convention this election cycle.

But what does that mean? What are the rules for this?

Interestingly, the rules change slightly each round of voting at the convention. If Donald Trump were to reach the required 1,237 delegates he will secure the nomination.

If not?  Check below.

From the New York Times: