The mainframe is the platform on which the product/application is being tested.
Mainframes are usually character based rather than GUI based
Test products on the mainframe as you would on any other platform.
Many of the tools available work on both GUI and Mainframe, such as Compuware's TestPartner for scripting and Compuware's QA Director for Test management.
The Mercury tools, Winrunner, QTP and Test Director also work.
Some vendors do have some specialized tools for mainframe testing such a Compuware's Hiperstation which only works on the large IBM mainframes.
Lynne, you're right, of course, but most people don't even know whether their LAN applications are actually host portals or not, even if they use 3270 emulators that look like real GUI's now. There is another side to host testing, based on transaction files for example, or to perform any batch processing that is not transaction based. And even that is only the beginning: there are a lot of specialized applications such as on-line banking and ATM/POS that require special testing facilities. Therefore, to Aravind, you may need to become familiar with a different processing paradigm to understand how you interact with such applications, but the good news is that testing principles are much the same.