Hi there: I was just searching about Gauge theories and stuff and I find it very confusing. My major complication is the classification. I'd like you to tell me some definitions and construct a "family tree". I guess it goes something like this: Gauge Theory: A Field Theory in which the lagrangain is invariant under the action of a Lie Group. Yang-Mills Theory: A Gauge Theory in which the gauge group is SU(N). QED: A type of gaugue theory in which the gaugue group is U(1) QCD cousin: without quarks: YM SU(N=3) QCD: A type of Yang-Mills in which N=3 (the 3 stands for the number of colors, doesn't it?)- How is the existence of quarks expressed in the class. of QCD? Please, I need this. I'd like you to complete this mini-tree (probably wrong) and include Electroweak theory, Super-Yang-Mills and stuff like that. Thankyou very much.