To ensure your bone structure doesn't bend in unnatural ways, you will need to contain the movement of some joints. Think about how an elbow works. You probably cannot bend your elbow in all directions. It is constrained to a certain range of motion.
You may also assign constraints to any joint in the Properties Panel. When you check the constrain box, you will see a small guide which shows the angles to which motion is constrained. Change the angle values in the Properties Panel.
Click on any of the bones and look in the Properties Panel. You will find categories related to the joint. Rotation is enabled by default, but uncheck the box if you do not want the join to rotate.