As with any other object, you edit your armature in Edit Mode Tab.
The set of bone editing tools is quite similar to the one for mesh editing. Blender also features a more advanced “armature sketching” tool, called Etch-a-Ton. The same tool might also be used in templating, i.e. using another armature as template for the current one…
One important thing to understand about armature editing is that you edit the rest position of your armature, i.e. its “default state”. An armature in its rest position has all bones with no rotation and scaled to 1.0 in their own local space.
The different poses you might create afterwards are based on this rest position. So if you modify it in Edit Mode, all the poses already existing will also be modified. Thus you should in general be sure that your armature is definitive before starting to skin and pose it!
Please note that some tools work on bones’ joints, while others work on bones themselves. Be careful not to get confused.