The 2011 and 2012 seasons are at the Pirates' option. Those years are worth at least a total of $16 million to Snell. There also are standard incentive clauses in those final two years.
If Snell is half the pitcher he can be, this is going to save the Bucs a ton of money in arbitration. It also potentially buys out a year of free agency eligibility at what appears to be a very reasonable price. Long term deals are risky, but like many of them, this one eliminates much of the risk with the inclusion of option years.