Tandem Mass Spectrometry, usually referred to as MS/MS, involves the use of 2 or more mass analyzers. It is often used to analyze individual components in a mixture. This technique adds specificity to a given analysis. 
The basic idea of MS/MS is a selection of a m/z of a given ion formed in the ion source, and subject this ion to fragmentation, usually by collision with inert gas (eg. Argon). The product ions are then detected. This is a powerful way of confirming the identity of certain compounds and determining the structure of unknown species. So MS/MS is a process that involves 3 steps: ionization-mass selection-mass analysis.

MS/MS could be performed on instruments such as triple quadrupole (QQQ), ion trap, time of flight, fourier transform, etc... The triple quadrupole is the most frequently used mass spectrometer for MS/MS, perhaps because of the cost and ease of use among other factors.