Your asking a bit much there my friend, entire textbooks are devoted to introducing calculus (my intro calc text is about 1500 pages). However, I am part the way through writing a brief introduction to differentiation is https://www.physicsforums.com/showthread.php?t=139690". At the moment it only covers very basic undergrad math, but I am considering extending it when I have a little more free time. However, if you were to spend a few minutes with google I'm sure you could turn up some reasonable calc resources. If you are intending to learn calc on your own or are looking for a reference for college then I would highly recommend getting yourself a hard copy of a good textbook.
Calculus is the study of the instantaneous infinitesimal differentials (or tiny bits of change). In differential Calculus it focuses of the rate of change of say, y compared to x. In Integral Calculus, it focuses of the changes in area under the curve, or any line.
Pretty hard to give a definition to calculus because it can be applied in so many different ways and thus will be defined differently by different people. I would say that it is the application of the idea of limits to the tangent problem and the Riemann sum problem (area under curve) like most people here have said. The best way to find out is to pick up a calculus book at your library, you could probably glance over the intro pages and get some sort of idea as to what calculus is.