A signal is defined as any physical quantity that varies with time, space, or any other independent variable or variables.
A system may be defined as a physical device that performs an operation on a signal.
If a signal takes on all possible values on a finite or an infinite range, it is said to be a continuous-valued signal.
A discrete-time signal having a set of discrete values is called a digital signal.
I don't intend to provide a full tutorial on DSP here but if you are interested the two good books to start are
Oppenheim's text on Signals and Systems is also very useful.
Of course the Internet contains a wealth of information but you have to know where to find it. Check out my links page for some good technical links.