> It is different to other ARM
ARM is just the CPU core (and things directly connected to this unit).
All other components (timer, PWM, interfaces, AD, DA, ...) are 
manufacturer specific.

> I always think it is AVR, not ARM.
> Like Devkit1207 board basedon STM32F207IGT6 and Devkit8500D board based
> on TI DM3730, can you tell why one is only 120MHZ, and one is 1GHZ.
You are messing up every possible processor family...  :-o

AVR is a small 8-Bit microcontroller family.
ARM is a 32 Bit CPU core.
The TI DM3730 is a highly specialized signal processor for video.

You CANNOT compare the one with the other. Its like comparing a Porsche 
with a bullet train or a childrens tricycle: each if it has wheels. But 
thats the only common they share...

Thank you very much for your information, you solve my confusedness.
And what you said is good that never make me confused.
The metaphor is good.

Well, I just saw the board Devkit1207 board from Embest and I thought 
that.

So can you tell me how to learn ARM from zero level?
I know a little about ARM, but it is just a little.
I want to learn from Zero.
Do you have some suggestions? Like how to learn and what do I need to 
learn and which OS is better to learn?

Please give me some help. Thank you very much!

STM32F207IGT6 is from ST, it is ARM Cortex-M3 processor.
DM3730 is from TI, it is ARM Cortex-A8 processor.
They are different processors based on different ARM cores.