About Tracks and Keys
A track describes the motion in an animation. A key is a milestone in time that signifies a change in the animation.
Tracks
There are three types of tracks:
• Container—Contains one or more container or item tracks.
• Item—Animates a part or camera. Each item has its own track.
• Property—Includes the keys that define the motion and properties of a part, subassembly, markup, section, or camera.
Keys
When you create or edit an animation, a new key is created each time you perform one of these actions on an object or content:
• Move
• Modify
• Create
Each frame of the animation sequence contains a key for each recorded item. The type of transition between keys is the Interpolation Mode. Select one of these kinds of transitions for each key to define the movement in your animation:
• 
Step—Changes only at the key’s position on the timeline.
• 
Linear—Transitions between keys at a constant rate.
• 
Spline—Transitions between keys at an increasing or decreasing rate.