What are differences between a scooter and a bike?

If you are talking about the non-motorised versions....

A bike has a saddle, and pedals, so that the rider can sit on the saddle and use the pedals to move the wheels. The pedals are connected to the wheels using a chain.

A scooter doesn't have somewhere to sit, but has a platform for the rider to stand on. There are no pedals. Instead, the rider has to propel himself or herself forward with one of his or her feet, pushing against the ground. The movement is similar to a skateboard, but the rider has handles to assist with steering.