Hi Brook,

MCS-51 is the familyname which Intel gave to the processors which all have the same CPU-core and thus the same instruction set.
Examples from the first time Intel introduced the family:
8031
8051
8032
8052
8751

Nowadays there are many derevatives from the 8051 (even from many different manufacturers), which all still have the same cpu-core and thus the same instruction set. These derivatives may contain for example internal FLASH-memory, I2C-hardware, AD- and DA-converters and much much more. In fact we can say they all belong to the MCS-51 family, even if not all of them are developed/manufactured by Intel.

Henk