beginners kits microprocessor arm
Most microcontrollers used by "beginners" are 8bit and have a simple instruction set, the arm microcontroller has a more complex instructionset, you can shift, add and move in just one instruction. that is what makes it so powerfull. ARM microcontrollers also have different types of operating modes. And a vectored interrupt controller, and a fast interrupt controller. ARM is develloped for speed and low power (mobile devices). There are some good books with a very simple introduction.