Sticking to their eight week release cycle, Microsoft has released two different Cumulative Updates (CU) for SQL Server 2008. The first update is SQL Server 2008 SP1 CU16, which is Build 10.00.2850. This CU has seven fixes by my count. The second Cumulative Update is SQL Server 2008 SP 2 CU6, which is Build 10.00.4321. It has 13 fixes included.
There is no Cumulative Update for SQL Server 2008 RTM, which is considered to be a “unsupported service pack” by Microsoft, meaning no more Cumulative Updates for the RTM branch will be released, and you will get limited support from Microsoft if you ever need to open a support case with Microsoft CSS. If you are still on the RTM branch, you really should be making plans to get on a newer branch of SQL Server 2008, preferably the SP2 branch. The Build chart below should hopefully make the relationship between the builds and branches more clear.
Remember, none of this has anything to do with SQL Server 2008 R2, which is a completely separate major release (at least according to Microsoft). SQL Server 2008 R2 is on a separate release schedule for Cumulative Updates (every eight weeks, but not in synch with SQL Server 2008), so you need to get Cumulative Updates for SQL Server 2008 R2 separately.
-- SQL Server 2008 RTM is considered an "unsupported service pack" as of April 13, 2010
-- SQL Server 2008 RTM Builds SQL Server 2008 SP1 Builds SQL Server 2008 SP2 Builds
-- Build Description Build Description Build Description
-- 1600 Gold RTM
-- 1763 RTM CU1
-- 1779 RTM CU2
-- 1787 RTM CU3 --> 2531 SP1 RTM
-- 1798 RTM CU4 --> 2710 SP1 CU1
-- 1806 RTM CU5 --> 2714 SP1 CU2
-- 1812 RTM CU6 --> 2723 SP1 CU3
-- 1818 RTM CU7 --> 2734 SP1 CU4
-- 1823 RTM CU8 --> 2746 SP1 CU5
-- 1828 RTM CU9 --> 2757 SP1 CU6
-- 1835 RTM CU10 --> 2766 SP1 CU7
-- RTM Branch Retired --> 2775 SP1 CU8 --> 4000 SP2 RTM
-- 2789 SP1 CU9
-- 2799 SP1 CU10
-- 2804 SP1 CU11 --> 4266 SP2 CU1
-- 2808 SP1 CU12 --> 4272 SP2 CU2
-- 2816 SP1 CU13 --> 4279 SP2 CU3
-- 2821 SP1 CU14 --> 4285 SP2 CU4
-- 2847 SP1 CU15 --> 4316 SP2 CU5
-- 2850 SP1 CU16 --> 4321 SP2 CU6