According to Wikipedia, "Tandem (or in tandem) is an arrangement where a team of machines, animals or people are lined up one behind another, all facing in the same direction."

A tandem bedroom is accessible by walking through another bedroom, not from a hallway.  As one bedroom is used as a walkway into the tandem bedroom, it is more walkway and offers less privacy.  It is an uncomfortable arrangement and typically found when a home has had an addition. 

There are also tandem garages, my Glendale listing on Justin has garage parking for eight cars.  One of the garages is a four car:  two front, two back, not four side by side, therefore tandem.  The other garage is four side by side.