That's a nice sum up Saltyyy.
Let me give you a run down on the majority of parts in an airsoft gun.
1) Barrel. You should know what these are, they typically range from 6.08mm wide to 6.01mm wide. Their length varies depending on the rifle. (I.e. a pistol's can be a few inches, when a sniper can be tens of inches).
2) Hop Up Unit. This unit is made up of a rubber "hop rubber/bucking", the plastic/metal "unit" that encases the bucking and an adjustment feature to increase/decrease the hop.
The hop unit causes the BB to backspin... this makes the BB "defy" gravity.
3) Gearbox. AEGs (Automatic Electric Guns) are run on a battery. The battery runs a motor, which turns a set of gears. These gears are encased inside a gearbox.
Also inside a gearbox is a piston, spring and cylinder. These parts create an airpocket which fires the BB. The gears pull the piston back against the spring and then let it go to fire.
4) Magazine. All airsoft guns are fed using magazines. These (as above) can vary between Hi-caps, Mid-caps and Low-caps. Hi caps are usually around 300-500rnds, but in Light Machine Guns (LMGs) they are often over 1000, upto 5000. Hi caps have to be wound as they operate on a clockwork system. Mids and lows are spring fed.
5) LiPos. You may hear about these. They are a type of battery that is VERY volatile if you have no idea what you are doing.
I suggest you stick to NiMH batteries. They are the best for a starter.
6) BBs. I am sure you know what they are, but you must make sure you have a good quality of BBs or they may damage your gun! the standard airsoft BB round (6mm) is 0.2 of a gram in weight. They can go up to half a gram for sniper rifles. Generally 0.25g BBs are considered to be the best of accuracy and range in a 300-350 fps gun.
Here's a little .gif image of a working AEG V2 gearbox. many M4/MP5 guns use this version gearbox: