There are four definitions to consider if you want the full answer. Consider a X-Y graph where X=time and Y=signal value
analog/Digital refers characteristics of the Y-axis
Continuous/Discrete-Time refers to characteristics of to the X-Axis
Analog/Continuous-Time means that the function (on the respective axis that the word describes) can take on any value...
1,2,3,4, 1/4, 2/3, 3.1415, a million and ten billionths. It doesnt matter.
Digital/Discrete-Time means just the opposite. The numbers on the respective axis cannot be any and every number. It can only be certain values like
{1,2,3} OR
{0,1} OR
{0, 1/4, 1/2, 3/4, 1}
For each set of numbers, the axis can only take on those values and ONLY those values. No numbers in between. For {1,2,3} its either 1 or 2 or 3. Not 0. Not 1/2. Just 1,2, or 3.
You can pair ip Digital/Analog with Discrete/Continous-Time any way you want to get four types of signals.
I will make some graphs. BRB. Very easy to see. The definitions come very easily.