Procedure cache is the old name for the plan cache.
Plan cache - cache of compiled query plans so that the optimiser doesn't have to regenerate them
Data cache - cache of data pages so that they don't have to be read off disk (slow) every single time they're needed.
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