1) In electronics, a signal is an electric current or electromagnetic field used to convey data from one place to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is the principle by which the early telegraph worked. More complex signals consist of an alternating-current (AC) or electromagnetic carrier that contains one or more data streams.
Data is superimposed on a carrier current or wave by means of a process called modulation. Signal modulation can be done in either of two main ways: analog and digital. In recent years, digital modulation has been getting more common, while analog modulation methods have been used less and less. There are still plenty of analog signals around, however, and they will probably never become totally extinct.
Except for DC signals such as telegraph and baseband, all signal carriers have a definable frequency or frequencies. Signals also have a property called wavelength, which is inversely proportional to the frequency.
2) In some information technology contexts, a signal is simply "that which is sent or received," thus including both the carrier (see 1) and the data together.
3) In telephony, a signal is special data that is used to set up or control communication. See signaling.