The 8051 was originally developed by Intel but there were many second sources. It has a Harvard architecture so separate code and data memory space and furthermore an external memory space. Also can do single bit operations which is useful for controlling IO pins. Although an 8-bit processor, the compilers (like Keil which is now owned by ARM) are quite good. Not a days if I was wanting a low end micro-controller I would go with an ARM Cortex-M as the 32-bit CPU just makes programming a lot easier. I recall the days I spent micro optimizing a 32-bit multiply or divide on an older 8-bit CPU.