What is a rover?

no Originally yes, the model now known as the Defender was originally designed and produced by the Rover Motor Company, the first Range Rovers were designed by them but didn't go into production...

a cool blue dude on Seasame Street

Land Rover Motor Company is the main automotive company. The Range Rover (and now the Range Rover Sport) are simply models under the Land Rover Marque. Land Rover cars including the LR2 (Freelander...

According to the Encarta World English Dictionary: "Rover" = Wanderer, somebody who wanders from place to place, never settling anywhere for long.

The difference is that the later is a SPORT.