"Enterprise" is the name of the RedHat distribution, after the last free one (that was RedHat 9).
So the name of the enterprise distribution of RedHat is now RedHat Enterprise Linux (RHEL).
ES and AS are two kinds (they mainly differ for the number of CPU supported). You have also WS that stands for WorkStation (the desktop version)
4 is the number of the version, and the number after the "." is the number of the update. So 4.3 is the 3th Update of the version 4 (that is RHEL 4 Update 3). You can have RHEL AS 4.3 or RHEL ES 4.3
This is the link from redhat site reporting a comparison