About ARM-processors see attachment. As far as I understand they are good for high speed. And that such 12 - I do not know. Probably it is digit capacity.
I have never heard of an ARM12, however ARM7 and ARM9 are 16/32 bit devices which can use two instruction sets, ARM and THUMB. The ARM set is complex, and the THUMB set reduced. They can switch between sets while running.
I've played with the Phillips LPCs, but they are harder to use than PICs or AVRs. There is a plethora of information, and you will learn a lot by Googling.
Regards,
Robert