Paintball Accessories can range from a variety of things.  They can be air systems (required), goggles (required), barrel plugs or covers (mandatory), hoppers/loaders (required), and remotes for your marker.

       This is a co2 tank.  Co2 stands for carbon dioxide.  This system is typically used by most amateurs or even intermediate players.  This is not used by any professionals.  

This is a high pressure air tanks.  It has compressed air in reaching possible pressures of 4500psi.  These systems are used mostly by intermediate players and even some professionals. 

These three tanks are for nitrogen.  These are the most expensive tanks you can buy.  They are usually outputs of 1450 to 1800 shots per tank getting filled.  This is what almost all professionals use.

This is an expansion chamber.  You can run your air system through this .  This little object/ upgrade can increase your number of shots by about 100.  This is a very good investment.

 This is an inline regulator.  You also run your type of air system through this.  This upgrade regulates or cleans your air, Co2, or Nitrogen.  It also does the same thing as the expansion chamber,  This is a great upgrade for your marker.

This is a picture of a hopper or loader.  They can hold various amounts of paintballs.  Some can reach about 230 paintballs, other are as low as 45.  The one pictured is electric and runs on a 9 volt battery.  This is one of the best hoppers money can buy.  

This is a mask to protect your face and eyes from paintballs that are going about 200 miles per hour.  This is one of the low - end masks.  Others may be more durable or they may also be more comfortable.

This is a remote.  You can connect the end of your air system to the end of your gun through a coil.  This can really take some weight off of your hands and put it on your waist.  This is a good investment if you play a lot.

This is barrel cover.  It prevents you from accidentally shooting someone when you are not playing.  These are required at all paintball courses.  Instead of having a cover, you may also have a plug that goes into the end of your barrel, they all work the same.  

This is a little belt of harness. that can carry canisters of paintballs or hold your air system.  These are used by every professionals.