I've been trying to do a project using Arduino, I need your help to know the basic differences between these. Thank you
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Ignacio gave a one-line answer - a more detailed answer could take books...
Each family of microcontrollers has its own architecture - registers, memory arrangement, memory addressing methods, and other things, which results in each family having its own instruction set and assembly language.
Newer processor families like AVR and ARM are likely to be more "compiler-friendly" and generally more flexible, than the 8051, which is a fairly old design.
The 8051 is an 8-bit processor (it processes an 8 bit data word at a time).
Many AVRs are also 8 bit, but some are 32 bit (handling 32 bits of data at a time), and ARMs are 32 bit, and are considreably more powerful than 8 bit processors.