Very General question on the idea of categories

Question

First I have to say that I am a physics student, I don't know very much about mathematics but I am really intrested and fascinated by it. Anyway, my question is a very General one, and even a reference to some good text on the argument would be an appreciate answer, but here we go: I've Heard and come across the word "categories" and "Groethendick universes" and some others for some time now and tried to Read some about it; unfortunately my already mentioned lack of culture in mathematics has posed some (insurmountable) obstacles to my understanding, and it prevented me to go further Than the Basic definitions and concepts of the field. I am not asking anyone to explain me any particular concept obviously, but I was Wondering if Maybe someone could at Least explain the relevance of those ideas (the one of categories to be precise) for modern mathematics, what was the problem that made them necessary and what kind of New tools they brought to the (beautiful) field of math. I hope it is an appropriate question. Thanks in Advance!