Define: Skeletons :-)

Abstract:

Joint, Bone, Skeletal Hierarchy;

Skeletal Animation

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Here is a description of one way to do it :
1. Your object will be in a default pose.
2. All the vertices are in their respective places.
3. You have an array of bones.
3. You have an array of offsets.
4. What's a bone? It is a rotation matrix. You only need a 3x3 matrix.
5. What's an offset? It is a XYZ value you use to translate your vertex.
6. A bone and its offset together make a 4x4 matrix.
7. Each vertex can have a certain amount of bones that influence it. 0, 1, 2, ...you name it!
8. For each bone that influences your vertex, you need to decide how much it influences. This is called a weight or blend factor. 1 weight per bone/offset matrix
9. In order to animate your object, you need to change the bones and offset matrix. Normally, you would not change the weights.[1st]

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A skeleton is comprised of a hierarchy of rigid pieces known as joints . In the game industry, we oft en use the terms “joint” and “bone” interchangeably, but the term bone is actually a misnomer. Technically speaking, the joints are the objects that are directly manipulated by the animator, while the bones are simply the empty spaces between the joints. 

The Skeletal Hierarchy

Head  
Spine_5  Spine_1
Left_elbow     Left_hand    Left_wrist
Right_elbow  Right_hand  Right_wrist


Pelvis 
Left_knee   Left_ankle   Left_toe
Right_knee  Right_ankle  Right_toe

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Joint

A joint allows relative movement within the skeleton. Joints are essentially 4x4 matrix transformations. Joints can be rotational, translational, or some non-realistic types as well..

Bone

Bone is really just a synonym for joint for the most part. For example, one might refer to the shoulder joint or upper arm bone (humerus) and mean the same thing...