Explicit save point :
Requires an end user action before a bounded or unbounded task flow creates a save point. For example, an end user clicks a button that invokes a method call activity that, in turn, creates a save point

Implicit save point :
can only originate from a bounded task flow if
1) A session times out due to end user inactivity
2) An end user logs out without saving the data
3) An end user closes the only browser window, thus logging out of the application
4) An end user navigates away from the current application using control flow rules (for example, uses a goLink component to go to an external URL) and having unsaved data.

good usecases and examples given by frank/biemond and on implicit save points
http://www.oracle.com/technology/products/jdev/tips/fnimphius/cancelForm/cancelForm_wsp.html?_template=/ocom/print
http://biemond.blogspot.com/2008/04/automatically-save-transactions-with.html