A lot of differences this is my take on them:
8i -> 9i - Real Application Clusters, PL/SQL enhancements, XMLType enhancements
9i -> 10g - ASM space management, tracing and diagnostics, DML error logging, async commit, more PL/SQL changes, table/column encryption, restore points
10g -> 11g - Probably best to point you to technet on that one.
You can find the links to the various versions here:
pls have a look at this docs
http://www.dba-oracle.com/oracle11g/oracle_11g_new_features.htm Differences between 10g and 11g