Service packs for which version of SQL? 7, 2000, 2005?
As for what differes, there's no single answer to that. Each of the service packs for each version fixes different things. There's usually a kb article for each service pack that lists the changes and fixes made.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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