When considering Ebikes, the motor system is what gives you the assistance. There are generally two different types of motor systems - hub drive and mid drive.
Hub Drive (Geared) motors drive by replacing the rear hub (middle of the wheel) with a motor which pushes the bike forward. These are usually controlled by a speed sensor or a torque sensor (which gives a more intuitive riding experience). They also generally will have a throttle as well which allows riders to use the bike like a moped if the rider does not feel like pedaling. Most hub drive systems use a 300W Bafang motor which keeps cost a little lower than other motor systems. Hub drive systems transfer power directly from the motor to the tyre meaning that wear on the drivetrain is kept to a minimum and maintenance is generally very minimal making them ideal for commuting.
Mid Drive motors drive from the bottom bracket location (in between the crank arms and pedals). They feel very intuitive as you are mixing your pedal power with the assistance from the motor. For this reason they are a very efficient motor system. Some mid drives may come with a throttle however the throttle system is usually not as effective as it may be on a hub drive. Mid drives come in a range of different motor variants, with power outputs from 40NM to 120NM. Mid drive motors are generally better at climbing than hub motors which makes them ideal for mountain biking.
Whats better? Both drive systems have their benefits. For commuting around areas that aren't overly hilly and maintaining a good pace, hub drives are perfect for their effective throttle and minimal maintenance. Mid drives on the other hand give a very smooth riding experience; delivering power just as you would expect from a regular bike. Mid drives are ideal for any sort of off-road or demanding mountainous applications.
Other Motor Types
Direct Drive - These are still a hub motor but unlike the 300W Bafang unit you find on most hub drive bikes, a direct drive has a much higher wattage and does not use a geared system to give gradual power but instead uses large amounts of power to build up momentum. These bikes often have a high top speed and are ideal on flat areas of land.
Electro Magnetic Hub Drive - These simple hub driven motors use magnets to drive the motor. They are a very basic motor system which makes them extremely reliable however they generally put out a low level of power and are very heavy due to the magnets.