A signal is an abstract element of information, or more exactly usually a flow of information (in either one or several dimensions). The signal can be either analog or digital. The former is a continuous flow of information in some framework (the signal value does not need to be continuous), while digital signals are constant within some constant temporal (or spatial) intervals.
A typical signal is sound, for example, speech sound whereby the signal carries the information of the spoken words, the identity of the speaker and for example, emotinal cues. Another typical signal is a radio transmission, which in turn, can carry the speech sound -signal. Both sound and radio signals are analog signals.
Digital signals are most usually either representations of analog signals within a digital framework (i.e., a computer), or computer generated signals.