What is Shadow Memory?

What is Shadow Memory? Can SO briefly shed light on how memory profiling tools use it?

From stackoverflow

• You either mean general term: http://en.wikipedia.org/wiki/Shadow_memory

  Or particular feature used in PC, which is better knowns as "shadow RAM": http://en.wikipedia.org/wiki/RAM#Shadow_RAM

• Shadow Memory is RAM that lives at the same address as ROM. It's used because it's faster and also because it can be modified (e.g. setting breakpoints).