It was an early Motorola 8 bit microprocessor. Deployed in 1974, shortly after the launch of Intel's 8080 microprocessor.
It had two accumulator registers 8 bit wide - A and B and an Index register - X, 16 bit wide, a 16 bit program counter and an 8 bit stack pointer.
Its design was the base for the 6502 microprocessor family, being one of the distinguished designs from the time. The 8080 gave birth to the 8085, and from there the 8086 family which continues to today.
The 6800 design uses the same bus to both memory and I/O which resulted in memory mapped I/O devices that lasted up to the 6502 based Apple II computers.
It also includes, among its 78 instructions, an undocumented Halt and Catch Fire (HCF) instruction, that would halt the microprocessor, and have it using its bus at maximum speed, which could actually burn poorly designed electronics around it.
Later on, there were created the 6801 and 6803 microcontrolers, that were a 6800 with some RAM, ROM and serial I/O built on chip, and later the much enhanced 6809 processor.