Two very different features. Both really for working across servers, thought it's possible to replicate within an instance
Start here:
Mirroring http://msdn.microsoft.com/en-us/library/bb934127.aspx
Replication http://msdn.microsoft.com/en-us/library/bb500346.aspx
Once you're read through that lot, if you still have questions please ask.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass