Stack[ ] shows the current evaluation stack, giving a list of the tags associated with evaluations that are currently being done.
Stack[pattern] gives a list of expressions currently being evaluated which match the pattern.
Stack[_] shows all expressions currently being evaluated.
You can call Stack from inside a dialog to see how the dialog was reached.
In the list returned by Stack[pattern], each expression is wrapped with HoldForm.
The maximum length of Stack[ ] is limited by $RecursionLimit.
Stack has attribute HoldFirst.
See The Mathematica Book: Section 2.5.11.
See also: Trace.