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