What is the difference between register stacks and memory stacks?

Registers are normally memory spaces internal to the processor or very close to it. They are generally faster than main memory and will be small in size and will hold very frequently used data.

Register stacks are a set of such register memory locations.

Memory refers to computers main memory outside CPU. It is used to keep data and programs. Memory stack is a series of memory locations.
The difference between register stack and memory stack is...