Most high quality E-Bike motor systems these days are a 'mid-drive' (or 'crank-drive') type.
Mid-drive motors are very efficient as the motor is allowed to spin fast (this is optimal for an electric motor) through the use of the bicycle's gears.
Bosch, Shimano, Impulse and Brose motors are all mid-drive systems.
This type of motor is located at the bottom bracket on the bike - the point where you push the pedals around. This location is the ideal position in terms of weight distribution as it creates a low centre of gravity between the front and rear wheels that helps keep the bike well balanced - the additional weight of the motor is unnoticeable upon riding the bike.
Mid-drive motors also usually sport a torque-sensor, which monitors the pressure you put on the pedals (hundreds of times per second) and delivers more or less power depending on the input of the rider. This gives a very natural riding experience, as the motor seems to magically amplify what the rider is doing, just when the power is needed.
Hub motors still appear on many E-Bikes. These tend to be more at the budget end of the scale (although there are several exceptions to this - the very high quality Swiss brand Stromer uses their own proprietary hub motor, for example), due to their more simple construction and operation. The can work very well on commuter type bikes, though mid-drive motors are a much better fit on off-road bikes. One down side of hub motors is that of weight distribution, since a large mass is located in the rear of the bike. This can make the bike fell rather back heavy, especially on steep hills or loose surfaces.