If no candidate has a majority after the first ballot, then the brokering begins. That means dealmaking, horsetrading, pledges, etc. until one candidate has a clear majority of the delegates' support. One or more candidates could drop out and throw support to the others, but there's no "run-off" vote between the top finishers.
It looks like it could happen this year, but it has not happened before in recent memory. Lots of people thought 1972 might end up brokered, as there were originally 12 (yes, TWELVE!) major candidates in the race at NH. However, McGovern scratched out a 5pt victory in California's "winner take all" primary, and went to Miami with enough to seal a first-ballot victory.
If you've not read it before, I highly recommend Hunter S. Thompson's
Fear & Loathing on the Campaign Trail. It's a very interesting and often hilarious account of the 1972 election, from New Hampshire to November 1972. The section on the convention is a must-read for any politics junkie who wants to see what a campaign is really like.