Currently I'm lightly obsessed with understanding Cortex-M debug internals. Hope to write a blog post that covers all the SWD, JTAG, AHB-AP, TPIU, ITM, ETM, DWT, FPB and probably several more silly abbreviations.

In the meantime here is a great post that explains ITM and ETM http://essentialscrap.com/tips/arm_trace/theory.html