An analog signal can have any value as opposed to a digital signal that could only have two values. A discrete analog signal can only have predefined values. For example the output of a DAC (Digital to analog converter). A continuous signal is an analog signal that can take any value and the distance between any two contiguous values is infinitesimal. Generally this is the kind of signals that you get in linear (or analog circuits)
A DC signal could result from a linear circuit, as at the output of a voltage regulator, Or at the output of a digital gate or at the output of a DAC. An also can be the input to a digital or analog system. So it depends on the application.