As well as the fully automatic green Auto mode, all DSLR's have several other very useful exposure modes. Aperture Priority and Shutter Priority Modes are two of the most popular semi-automatic modes. But what many people don't realise is that generally speaking, these two modes will give you exactly same exposures. For example, take a look at these settings which show equivalent settings (assume that the current light level is giving a reading of aperture f4, shutter speed 1/250th of a sec):-
Every one of the above settings will give you the same exposure, for example, compare f5.6 at 1/125th of a second with f11 at 1/30th of second. f11 is 2 stops smaller than f5.6 (so less light is let in), and so to compensate for that, the camera chooses 1/30th of a second, which is 2 stops slower than 1/125th (shutter is held open longer).
Also, the only difference between aperture priority and shutter speed priority, and the clue is in the name, is which setting you have direct control over, either the aperture or the shutter speed.