For details and to try to explain the above, the complex scalar field is a useful example. The complex scalar field has an action S=∫d4x(∂μϕ∗)(∂μϕ)−V(|ϕ|). There is a global, continuous symmetry to this action -- an overall phase. That is, if one replaces ϕ→eiαϕ, then the action does not change. This type of change is called a gauge transformation. The symmetry group of this transformation is the Lie group U(1). http://www.quora.com/What-is-Gauge-Theory-%28intuitively%29 [Broken] Can anyone explain this to me as simple as possible? I dont really need to know it but I want to know what is all the symmetry about. I roughly know what does symmetry mean in physics terms but what is up with the local symmetry or global symmetry? And what is symmetry group? From my understanding, symmetry of a physical system is a physical or mathematical feature of the system that is preserved or remains unchanged under some transformation. I know I am way too early to even touch this but I am really curious about it.