Hi,
i am making a robot which would be having a weight of not more than 10Kg.it is a four wheeled robot with chained differential drive and has two motors in all.i want it to have a good speed and a good torque.can any one tell me which motor will be best for it,and y that motor is best??

 

Since your overall robot speed will be determined by your wheel characteristics, and the hand-waving description of your speed and torque requirements, I'll give you this link, it will help you determine which motor is best suited for your robot.

I'm not sure that any of those motors will suit your needs...10kg is quite a bit...

 

Here's another link that is a calculator for robot speed/ rpm / wheel diameter. In conjunction with tshuck's resource that he provided to you, you may find it useful.