Well, I know someone probably gave a decent answer, but here is mine (note I am a Sun Solaris UNIX Admin).
BASH is actually an acronym for Boure Again SHell. In other words BASH shell was based off the UNIX Bourne Shell, and is basically an upgraded more modern version. Bourne is USUALLY the default Unix shell, but not always. There are many different shells in Unix all having pros and cons, and as such different versions of Unix use different shells for their default shell. But in many cases, and in all of SUN Solaris UNIX, bourne shell is the default installed shell. Note that this can be changed by the system administrator, and/or by the user. I personally prefer csh or C Shell, but ksh (korn shell), and tcsh (updated version of csh) are also common defualt shells. BASH is just finally catching on in the UNIX world, as it is the primary shell in LINUX, and many UNIX users are starting to install LINUX for home use. Bash has been ported to all the major UNIX distributions, but it is not yet a default package in many of the versions.