What's the difference between ETM and Debug?

Try this:

ARM Information Center

HTH,
Pete

Thanks very much for your answer!

And I find the answer here:

http://www.arm.com/zh/products/system-ip/debug-trace/trace-macrocells-etm/

When it comes to debug or to optimize an embedded system, embedded developers
have two main options available to them:

1. Conventional debug: typically setting breakpoints and/or watch points
   to halt the processing unit and from there use a debug connection to examine or
   modify register or memory and single-step to understand how the program
   works.
   
   While enabling developers to control the execution and debug their
   code, conventional debug has several disadvantages:
   
   
   • Intrusive: debug halters the behavior of the system
   • Requirement to stop the processor, for some applications it may not
     be possible to stop the processor (e.g. hard-disk, automotive, …)
   • Non real-time: cannot be used to debug software operating real-time.
   • No performance visibility: cannot observe software performance.
2. Real-time trace: when the system is running, trace macrocells collect
   instruction and / or data transfer, compress this information and deliver
   off-chip in real-time or on-chip for post processing.
   Trace is post-merged
   with source code in development workstation for later analysis.