The most basic are spring powered replicas. They use a hand operated mechanism to pull a piston against spring pressure and lock it back, whilst loading a BB into the hop unit. The trigger releases the lock and allows the spring to push the piston through a cylinder, rapidly expelling the air inside the cylinder through the hop unit and accelerating the BB down the barrel.
This category includes cheap single shot pistols like the two tones you will see in hobby shops, but also include skirmishable weapons, such as shotguns and sniper rifles.
Spring shotguns are a viable alternative to an AEG, they can be effective and fun to use but require a higher skill level than automatic replicas.
Sniper rifles are the most common spring weapons found at skirmishes. These may fire BB’s to the same or greater range than a regular airsoft replica, depending on many factors, but the main reason to use them is that they can fire with greater accuracy than other airsoft replicas. It is advisable to forget about sniper rifles until you are able to use an AEG well. Most new players harbour a fantasy of being an elite sniper, but new players will fail miserably if they try and learn how to use a bolt action weapon before they have the skill to use an assault rifle in airsoft.