An 8-bit microprocessor. It was released in July 1976 with a 2.5 MHz clock rate. The Z80 was a much improved Intel 8080 (as was the Intel 8085). It also used 8-bit data and 16-bit addressing, and could execute all of the 8080 op codes as well as 80 new ones, instructions that included 1, 4, 8 and 16-bit operations and even block move and block I/O instructions. The register set was doubled, with two banks of registers (including A and F) that could be switched between. This allowed fast operating system or interrupt context switches. It features 3 types of interrupt mode.
The Z80 also added two index registers (IX and IY) and relocatable vectored interrupts (via the 8-bit IV register). Like many processors (including the 8085), the Z80 featured many undocumented op codes. Chip area near the edge was used for added instructions, but fabrication made the failure of these high. Instructions that often failed were just not documented, increasing chip yield. Later fabrication made these more reliable.
The thing that really made the Z80 popular was the memory interface - the CPU generated it's own RAM refresh signals, which meant easier design and lower system cost. That and its 8080 compatibility and CP/M, the first standard microprocessor operating system, made it the first choice of many systems.
In addition to the original Z80 (2.5 MHz) there are the Zilog Z80A (4 MHz), Zilog Z80B (6MHz) and Zilog Z80H (8 MHz) versions. The popular Hitachi HD64180 processor family adds peripherals and an MMU to the Z80.
The Z80 was used in the first Nintendo Game Boy. A Sharp Z80 work-alike was used in the GameBoy Color, running at 4 MHz for GameBoy software or at 8 MHz for Game Boy Color software. The Z80 was used in the Sega Master System and the Game Gear. It was also used in the Sega Genesis for hardware reverse compatibility with the Sega Master System through a special cartridge.