Linux started out as a UNIX 'clone' that would run on cheap x86 hardware (UNIX used to only run on proprietary chips made by the various UNIX vendors). Solaris is Sun's (crappy IMO) version of UNIX. Solaris is one of the few UNIX OS' to now run x86 hardware.
If you google 'linux vs. unix', you will find plenty of good articles/info about the topic.
Red Hat is a vendor that produces an enterprise class version of Linux. They have a few different classifications of their Linux offerings; AS=Advanced Server, EL=Enterprise Linux, etc. They've changed their naming scheme a few times so it will be easier for you read here
to figure out the info you wanted.