Actually microcontroller are classified depending upon their architectures CISC or RISC PIC follows RISC hence it has only 35 Instructions to manipulate whereas Motorola, AVR, Fujitsu and other Microcontroller follow CISC type of architecture which has 136 instruction set you need to understand the basic architecture operations first to get into more details.
The other classification of Microcontrollers could be done using based on their features(peripherals) and processing power.
THese days we are getting Microcontroller which can be configured as per our need for like Psoc for more details log on to www.cypress.com
Let me know if you need any manual for architecture details you get at any web site.
hi
yaar ur question is a very general it is not basing on whether it is cisc or risc to choose a controller it always depend s on the application u do.if u require more number of devices to be interfaced u should check for a controller that has max no of IO pins ,similarly if devices to be interfaced are less and should be low cost u can choose one like atmel 89c51.
Maybe it depends on your country too. Here, in my country the 51 series are more popular than the PIC or AVR series so we choose 51 series because of the cost.
Here we first learn how to use 51 series than for difficult application we use AVR.