Analog circuits have a range of voltages, rather than just two as in digital logic.
Analog circuits work with continuously variable voltages, in contrast with a series of discrete voltage levels. (While Wikipedia defines this as time-continuous, discrete-time systems are often considered analog as well: for instance, a sample-and-hold circuit, or switched-capacitor filter.)
Analog circuits are used in the processing of sensor inputs with signals proportional to a physical quantity (see sensor). The complete circuit, up to the output, may be analog, or in a mixed-signal (analog and digital) circuit the signal may be converted to digital after the analog input stage.