If you have a bone in your model that you will only want to turn or move in certain directions, Manipulator Options in the bone's properties can make the bone present only those axi when you use the Rotate or Translate manipulators.
This video uses a gun (swings only up and down) on a tank turret (turns only side-to-side) as an example case.
link in Post #14
Other examples of things that typically rotate in one axis only would be fingers, knees, elbows, eyelids, wheels, gears, doorknobs, levers...
Examples of things that translate in only one axis would be sliding doors, pistons, curtain hooks, double-hung windows, telescope segments...
Note that this only limits the options that the manipulator tool makes visible. It is still possible to manually do whacky things by dragging the bone without the manipulator.