CO2 can be supplied by fish, by the decay of organic material, or by direct supplementation. I hope to write a CO2 FAQ page soon, but for now this brief summary will have to do. If you want fast, aggressive plant growth or to grow a plant that requires high light and fast growing conditions, you will need to add CO2 to your tank unless you have a very, very high fish load. I call the remaining sources magic because there are no examples I have seen that have worked reliably and cost-effectively for a wide selection of users. They mostly fall in the category of snake oil.
Yeast can be quite reliable and has very low startup costs.
The downside is that it is a constant source of annoying drudge work. If you like washing dishes, you may love yeast as a CO2 source. If you are lazy like me, you will appreciate the convenience of compressed CO2. Once every couple of years, you get your tank refilled. Being lazy, I only stock and support compressed gas systems .