UNIX is primarily server technology, although many years ago it provided 'terminal' like access for most of the end users. There are some UNIX workstations, but they are typically used by software programmer types to compile applications hosted on UNIX servers.
But Linux has something that UNIX hasn't been able to market well yet, and that is the ability to run on Intel chips and motherboards, although it hasn't yet made much progress into the desktop market because of it's clumsy (immature) interface and lack of accross the board applications.
For that reason, Linux has made most of it's gains devouring the UNIX server market instead of the desktop market dominated by M$ which offers near 100% compatibility with user applications such as games and other peripheral devices. Once UNIX property owners SCO started seeing this errosion of their market, the started investigating Linux, and found the stolen code.
At least that is my understanding.