Here you go. My version of CO2 101.
As you probably know, CO2 is needed by plants for photosynthesis. In the water, the plants are unable to get CO2 from anything except possibly bacteria and the livestock breathing in the water. This, unfortunately, usually isn't enough and requires that CO2 be injected either through DIY or pressurized means.
A pressurized system can been done using a CO2 cylinder, CO2 regulator, needle valve and CO2 tubing. With this, you have the most basic CO2 pressurized system, but it is completely manual.
Most opt for a semi-automatic version, which includes a solenoid. A solenoid attaches to the regulator and acts as a springloaded valve that opens and closes when it is on or off. This allows for the CO2 to be set on a timer usually in sync with your lights to aid in photosynthesis.
For the more adventurous (and wealthy), an automatic system will further include a ph controller, which will turn on and off the solenoid depending upon the pH readings of the water.
Now with a DIY system, all you need is a decent sized bottle. Most common are 2L bottles or large juice bottles (I use apple juice). The bottle is filled with a yeast recipe consisting of warm water, yeast and sugar. The yeast will eat the sugar and create CO2. The bottle should be sealed leaving only an opening for CO2 tubing. Voila, you've got homemade CO2. This method is very cheap, but create inconsistent CO2 and requires that the recipe be remixed about every 2 weeks.
The last thing you would need to worry about is how the CO2 is diffused. There are several methods. I think the most popular are reactor, bubble ladder, powerhead and diffuser. Each of these will work and its really a matter of preference, size of your tank and method of CO2.
I hope this helps! Heck, by the time I finished writing this, there will probably be 10 posts... we'll see.