While signals are the means by which information is propagated, systems are the environment in which the signals are propagated. When we consider signals, we have no choice but to consider the system in which those signals exist because the system will have some sort of effect on the signal. That effect may be positive (amplifying or filtering the signal) or it may be negative (adding noise or attenuating the signal). Look at the block diagram below which is a communication system model. We can see the strong relationship between the system (the boxes) and the signals (the lines connecting the boxes):
As the signal moves through the system it will be affected; sometimes those effects are by design (like the effects of the transmitter on the signal) and sometimes those effects are just incidental (like the introduction of noise by the environment).
The subsections in this section will describe more about what a system can do to a signal.