control systems is a very vast field. it deals with single-input-single-output (SISO) systems and multiple-input-multiple-output (MIMO) systems. it deals with linear systems as well as discrete systems. it deals with time-invariant systems as well as time-variant systems. it deals with deterministic as well as stochastic processes. the components of a control system could be lumped or distributed. and the list goes on.
so basically control systems is a study of systems in general and their design, modelling and simulation.
the control systems course usually taught at the undergraduate level starts with laplace transforms, transfer functions, poles and zeros, mathematical modelling of systems, transient analysis, root locus, bode analysis, nyquist analysis etc.
it is really fun!!!!