AEG's (Airsoft Electric Gun) have an electric propulsion.
This ensures that the bb's are being fired automatically, so you do not need to load the weapon after each shot. It therefore ensures a realistic experience.
Sportline: The Sportline replica's are buget based. These often have plastic bodies and lower quality gearboxes. The advantage is a lightweight replica with low costs!
Proline: The Proline replica's are the more expensive class. The most replica's you see, are Prolines. With a metal body, these replica's are more durable and will last longer. The gearboxes are often high quality. The advantages are that these replica's can take a beating and will last you a long time!