The Z80 was Zilog's attempt to compete with the Intel 8080. It's main competitor was the Intel 8080 and Intel 8085. I believe the Z80 had 16 registers, but only 8 could be addressed at any one time. It came in many speeds, with the 1 and 2 MHz versions being popular in home computers, and the 4 MHz version being popular in bigger CP/M based systems.