You can read about the caches in the Memory Architecture section of the concepts guide:

http://docs.oracle.com/database/121/CNCPT/memory.htm#CNCPT007

But basically you have:

- The buffer cache. This stores data blocks read from disk. You can split this into separate pools if you wish
- The Smart Flash Cache: If you have flash disks, you can configure this as a second level cache
- The shared pool. This contains the:
- library cache: stores shared SQL & PL/SQL structures
- data dictionary cache: this holds metadata about the database
- result cache: holds the results of the SQL result cache and PL/SQL function cache

"Are there any usefull SQL queries which can help me to see some usesull performance data"

What precisely are you looking for here? What is "useful performance data" for you?

You can find details about the buffer pools in v$buffer_pool_statistics:

http://docs.oracle.com/database/121/REFRN/GUID-1E914318-C648-4A72-9AA9-711F8CAA92FB.htm#REFRN30033