I sometimes hear these mistakenly referred to as the dollar tables, but their actually views. These are the system views that are named V$something. Because they are in the SYS schema, you'll sometimes see them called properly as SYS.V$something. The views are of interest to SAs because they contain dynamic performance data. When looking at system views you might also see GV$something views, these are for RAC installations were you want to see the Global View.
You can learn about all of these views in the Oracle Database Reference.
http://download-west.oracle.com/docs/cd/B14117_01/server.101/b10755/dynviews_part.htm#i403961