The document discusses the classification of signals and systems. It defines signals and provides examples. Signals are classified as continuous-time or discrete-time. Continuous-time signals vary continuously while discrete-time signals are sampled. Both types of signals can be further classified as periodic or aperiodic, deterministic or random, energy or power signals. The document also defines causal and non-causal signals. Systems are classified as linear or nonlinear, time-variant or time-invariant, causal or non-causal, and stable or unstable.