Service packs are the means by which product updates are distributed. Service packs can contain updates to improve system ...

Service packs are the means by which product updates are distributed. 
 Service packs can contain updates to improve system reliability, program 
compatibility, security, and more.  For more information on Microsoft 
Service Packs, see

http://support.microsoft.com/efault.aspx?scid=fh;EN-US;sp.