Microsoft has released SQL Server 2012 Service Pack 2 Cumulative Update 5, which is Build 11.0.5582. This Cumulative Update has 27 fixes in the public fix list. This is the build that you want to be on if you are running SQL Server 2012, since you should be on the SQL Server 2012 SP2 branch by now, at least in my opinion. There are a number of pretty significant fixes in this update.

Microsoft has also released SQL Server 2012 Service Pack 1 Cumulative Update 15, which is Build 11.0.3487. This Cumulative Update has only one hotfix in the public fix list. As I said above, if you are running SQL Server 2012, you really should be on the SP2 branch, even though SP1 is still supported.

There is no corresponding CU for SQL Server 2012 RTM, since SQL Server 2012 RTM is not a supported Service Pack. You really should not be on the SQL Server 2012 RTM branch anymore.