Hop Up Bucking is the part that puts back spin on the bb. Marui guns come with decent ones but other companies make better ones. Systema, Guarder, Prometheus and Fire Fly are the best. Fire Fly makes a special one with 2 nubs in the bottom to create a better spin.

Hop Up Chamber
The Hop Up Chamber can be changed for more consistency. The stock plastic ones in Marui guns are ok but are not as good as ones made by Systema. King Arms, Prometheus, and Madbull's Ultimate Hop Up are said to be the best.

Barrel
Some people change to barrels longer or tighter than their stock ones. The tighter the barrel the better. Don't get one too tight if you use bad quality BBs, or it will jam. Most AEGs come stock with 6.08mm diameter barrels. Systema make 6.03mm, Star make 6.05mm, Guarder make 6.04mm, prometheus make 6.01mm, Deepfire makes 6.04mm, Madbull makes 6.03mm, KM makes 6.04 with a special TN coating, which increases the FPS by 10+-, Dees Custom make 6.01mm, JBU makes aluminum 6.01mm/6.03mm, and PDI the best make 6.01mm from steel.

Spring
The stronger your spring, the higher the velocity the piston can be forced forward at. Don't fit a high powered spring with standard internals as you are likely to damage the gears or piston. Guarder and PDI springs are best. Systema can crack easily but prometheus is also good, although their "unique" coloring wears off after a while, damaging your gears. Most springs besides PDI are rated in metres per second, so a M100 spring is 100 metres per second or 328 feet per second.

Spring Guide
This stops the spring from jumping into the gear case. Marui guns come with plastic guides as standard but Guarder, Systema and Prometheus make steel ones. If it has spacers it will increase your muzzle velocity.

Cylinder
The cylinder is the area in which the air is compressed before being forced down the barrel. Bigger cylinders let you use a longer barrel. You can also get telfon-coated cylinders which will give a slight increase of Rate Of Fire (ROF), to my knowledge about 100 rounds per minute. This part isn't upgraded much as most guns come with good ones. For a budget gun one get a teflon coated Area 1000 cylinder.

Cylinder Head
Changing this part will either silence your gun (if you get a Systema or Angel damped head) or give a better air seal. Guarder makes excellent ones with two 'O' rings.

Air Nozzle
The air nozzle is the part that pushes the bb into the Hop Up Chamber. It is also needed to seal the airflow from the cylinder head to the BB, so some have internal 'O' rings like Guarder.

Piston
The piston compresses the air in the cylinder. It is essentially a flat-toothed gear. It is wound back by the motor and then forced forward at high speed by the spring, compressing the air in front of it. It therefore is subject to tremendous pressure. Metal pistons are good for Semi-Auto AEGs but if fired in Full Auto they may strip the gears. G&P, Guarder,and Deep Fire make good ones. Prometheus is OK with some mods. The Super Core Piston made by System (not to be confused with Systema) is probably the best of all, but is hard to find outside Japan.

Piston Head
The piston head seals the air into the cylinder. You can get good and bad ones. Classic Army and most clones generally have great piston heads. You can also get silent ones to quiet your gun and bearing ones to increase the muzzle velocity. G&P, Systema, Guarder, and Prometheus make good ones.

Gears
The gears are a very important and expensive part of the rifle. Don't be tempted to buy cheap gears, as you will probably end up paying a lot more to fix the gearbox when they break. If you are upgrading to high powered springs or hi-speed/torque motors opt for steel gears. If you want high ROF you can buy high speed gears from Prometheus, Systema and Guarder. Systema, Guarder and Prometheus make high torque as well. Most of these are helical which means the teeth are at an angle. If you use these you need a half toothed piston. They are also harder to shim correctly. You have 3 gears, the sector, bevel and spur gears.

Bushings and Bearings
The bushes keep the gears in places and are what the gears rotate on. If they break the gears will probably strip. The bigger the bushing the better. Most guns ship with plastic 6mm bushes fitted. If you upgrade go for metal bushes and if you are looking for higher ROF or torque, get a new gearbox casing that will take 7mm or 8mm bushes. Bearings on the other hand, have small metal balls inside of them that increases ROF even more. 6mm and 7mm bearings can't handle strong springs, while 8mm and 9mm can since of their greater size.

Shims
The shims are used to keep the gears aligned and correctly meshing. Incorrect shimming will lead to increased gear wear or failure.

Tappet plate
The tappet plate pulls the air nozzle back to let the BB in. If it snaps your gun will not feed. TM stock and ANGEL are the best. A Chinese brand called World-Element also makes decent ones for a good price.

Selector Plate
This part rarely breaks. It can be upgraded for more electrical stability.

Anti-Reversal Latch
This latch stops the gears from spinning backwards and damaging the gun. Systema and Prometheus make aftermarket versions.

Cut Off Lever
This stops the gears so you can fire Semi-Auto. If it does break or wear you will only have Automatic fire.

Trigger mechanism
If you fire too much on Full Auto the mechanism can burn out and your gun will cease to fire. Systema and Guarder make aftermarket versions. Keep the contacts clean to avoid short-circuits.

Sector Chip
Guns don't come stock with these but they essentially keep the tappet plate back longer so the BBs feed better. They have no negative side effects and will not break.

Motor
You can opt for hi-speed or torque. Don't put a hugely powerful motor in a standard gun and hope it will work. You will strip the gears or piston. The Systema Magnum and Turbo are excellent. The G&P M120, M160, and M170 are great as well.

Battery
The higher the voltage, the higher your ROF will be. The higher the Milliampere-Hour rating (mAH), the more BBs you can fire per charge (as a rule of thumb for a fairly standard gun, 1 mAH = 1 shot). Intellect, G&P, Sanyo and Elite are great. Most chinese guns come with poor batteries. Do not go for the highest power battery you can find straight away, as the voltage may be too high for your current setup and may damage components. Search the net and airsoft forums for advice on battery selection. 12volt= INSANE performance. Many guns even upgraded cannot handle these batteries.

Stock
Most stocks can be changed for tactical ones