Welding is a life long learning process. Anyone can throw down a few clean lines with a mig welder and have fun doing it, but knowledge of metalurgy is crucial to creating welds that work. There are a few great forums online between Miller, Hobart and Lincoln to really find the info you need but I just found another great resource within ESAB, that is far simpler to navigate. They have a "University" section on their page and if you click into the links there is very easy to find information on the tabbed sections about anything you might need from filler metals to Spray Arc Welding. The images below were found by doing a simple image search with google.
ESAB Welding University