What are these V$ tables?

There are a number of tables/views beginning with V$ that hold gory details for performance monitoring. These are not guaranteed to be stable from minor release to minor release and are for DBAs only.
There are usually no real underlying tables (unlike SYS.OBJ$) and are dummied up by the RDBMS kernel software in much the same way that UNIX System V.4 dummies up the files in the /proc or /dev/proc directories.
If you have any code depending on these (and the widely used tools supplied by Oracle but unsupported are in this category) then you need to verify that everything works each time you upgrade your database. And when a major revision changes, all bets are off.