Tracking Shot
A tracking shot is created by mounting the camera on a dolly. It's mainly used to follow an actor or an object. It can be used to follow characters in a scene or to get a bigger picture of a setting. However this type of shot can be hard to get right due to having to keep the camera at a steady pace and in time with the object/actor. It can also be used above an actor on a crane as well.
Here my group created a tracking shot tutorial on showing how it should and shouldn't be done:
A clip from Kill Bill showing an extensive Tracking shot from above and following from eye level.(Sorry if you can't understand the language - but as it's the tracking we are looking at I thought it didn't really matter...) :)
A scene from Footloose showing camera following dancers feet:
Arc Shot
This shot is mainly used when you want to make the camera go behind and around a character. It's often used in dance scenes as the actors are going round in circles etc. This dolly used for this type of shot hasn't got a track but instead three wheels so it is able to move freely around in circles.
This opening scene from The Black Swan has many tracking shots, a few arc shots in it as the camera is moving around her:



