Session based testing is a method to increase the structure and manageability of exploratory testing. It provides a high degree of flexibility and utilizes the talents of the experienced tester. It also minimizing the amount of time spend on documentation and other non testing activities.
With session based testing the Sessions have replaced the test cases as the central unit in testing
An uninterrupted period of time spent testing, ideally lasting one to two hours. Each session is focused on a Charter. Testers can also explore new opportunities outside the Charter during this time. The tester creates and executes test cases based on ideas, heuristics or whatever they find useful in guiding them.
A charter is the focus or goal of a test session. They can be created from design or specification documents or by brain storm sessions with developers, designers and stakeholders. A charter can also be created as a result of a previous test session.
The test results are documented in session reports. The reports will at least include; test notes (what and how), identified issues and ideas for future test sessions. Requested metrics can be generated from the session reports (issues found per session, test time spend per area, sessions per day etc.)
The changing list of Charters which remain to be executed
The wikipedia page is a good place to start to find more information: http://en.wikipedia.org/wiki/Session-based_testing