There are 2 different types of ebike motor, hub and mid-drive and both have advantages and disasdvantages. Hub motors are built into the front or back wheel. They require virtually no maintanance as all the components are sealed in the hub. You also have 2 independant drive systems, motor and pedals/chain so if one fails, you have the other to get you home. However, as they are fixed gear, they are more suited to flat ground rather than climbing hills and the centre of gravity shifts to the front or the back wheel.
Mid-drive motors are located in the centre of the bike giving better weight distribution and better handling. They drive the chain and utilize the rear gears which means the motor can run at optimum revs whatever speed you're doing. This adds up to amazing hill climbing ability and increased battery life/range. It's also a lot easier to remove the wheels for transport or puncture repair. Maintanance wise, the motors are similar but because the mid-drive uses the bikes chain and gears, they get a lot more work than with just human power alone so a well maintained drive train is important.
Which one is right for you? Only you can say. Personally, I've used both and have stuck with the mid-drive. I love the hill climbing ability, better weight distribution and being able to easily remove the wheels to put in my car.
Buy your mid-drive motor kit from here.
Buy your hub motor kits from here.