Buffer pool

Hi all,
some questions of understanding about cache and buffer.

So i know that there is the plan cache containing plans for procedures, queries... I can "see" the content of the plan cache using the sys.syscacheobjects or sys.dm_exec_cached_plans. And i can clean the cache using "DBCC FreeProcCache".

I know there is a "DBCC dropcleanbuffers" wich "Removes all clean buffers from the buffer pool". But what i exactly is the buffer pool? And what  is in it?

Any help would be great :)

Frank