There are basically two types of cracks. One is a movement crack, wherein the movement of the wall (usually associated with the movement of the foundation) causes certain stresses in the plaster and one side of the plaster gives way to accomodate the movement.
The other type of crack is the natural expansion and contraction of the ages. This type of crack is easiest to fix because it will be quite a while before it will come back.
To determine the type you need to examine the crack carefully.
Movement cracks are normally found in the corners where one wall meets another, above and below the corners of windows and doors or large, sometimes very long, cracks that go in a diagonal from a corner and up and across the wall. This is usually very deep also and usually exposes the lath below.
The other type of plaster crack is the old-age crack. This is usually a very shallow crack that can be seen most anywhere on the wall. Many times not even going the length of the wall (or ceiling for that matter). You can see these types of cracks going up, in the middle of the wall, from the base board and stop just a few feet or more. You can also see this type of crack right smack-dab in the middle of the wall and not go anywhere in particular. These are seen frequently in ceilings that have the original "button-board" plaster that is very similar to the present day "Blue-board". These cracks will go along the edge of one of the boards or maybe encircling certain boards entirely.