database buffer cache, library cache

Database buffer cache:

LRU is used.
When a query is proccessed,Oracle looks database buffer cache for any block it needs.
If block not found in database buffer cache ;server reads block from datafile and places copy in database buffer cache.

Library cache:

Stores information about most recently used SQL and PL-SQL statements.
SHARED SQL: Stores and shares execution plan,parse tree,...
SHARED PL-SQL:Second time same/identical SQL advantage to available in

Oracle will look for the execution plan and check the statement for syntex and syntatical correctness in shared pool first and then execute the statement and look for buffer caches for same data .