Evolution is a theory and a fact. Creationists like to say "iTs JuSt A THeOry" because the layman does not know that theory and "scientific theory" are different. It's the equivalent of saying "gravity is just a theory". Being a theory means it has substantial evidence to be used as a model to explain a scientific system. 

Anyways, evolution is the change in allele frequency in a population over time. Some examples that are used as evidence of evolution include: antibiotic resistant bacteria, ring species, comparative embryology, comparative physiology, comparative biochemistry, and the list goes on.