Simply put a CO2 system is a method of introducing CO2 to your aquarium. Live plants need 3 things to thrive in any situation, Light,Food, and CO2. Plants are the opposite of animals, as we breathe we take in Oxygen and in return we expell CO2. Plants on the other hand take in CO2 and return oxygen.
Now in most aquariums fish and some decaying foods will release CO2 into the water. While this is alright, those that want their plants to really do well will add fertilizer, extra lighting and you guessed it CO2.
While there are several methods of introducing more CO2 to your tank ranging from a few dollars to costing over 200.00, you will find that there are many economical ways to achieve this.
One way is to build you your own Co2 reactor and run the air line to your tank into an air stone. The cost of this method will run you around 10.00 to 15.00 usd to build and the cost is reasonable to replenish. There are also systems that you can buy at the LFS that run about 30.00 or so and they offer refills for them at about 5.00 or so for three refills if I am remembering right.
On the high end you will find people that have spent the money for a regulator and needle valves and the tank to introduce Co2 into the tank.
While all these methods work its really up to the aquariast to decide what level they want. I for one is what Allie calls a cheepo and I build my own and its easy to do.
Hopefully this will be of some help to you in your seach for the perfect aquarium garden
Les