RAC and Grid Computing , are these similar or different or one is subpart or other ?

RAC existed as OPS (Oracle Parallel Server) back in Oracle v7. So the concept of a clustered database
(shared everything database running across several servers) is not new.

What did change was the RAC s/w stack and architecture - as did HPC (High Performance Computing) in general.
The clusters we build today are very different from those we build back in the 90's.

So as the h/w technology changed, so too did the s/w technology for clustering. And Oracle (mostly for marketing reasons IMO)
choose the term Grid to describe their cluster s/w stack.

Grid in Oracle-speak basically means a s/w stack that provides the basic foundation for a clustered system.
RAC happens to be the house most frequently build on this foundation.