Analog signal vs. Alternating current
I think you are confusing two different concepts.

DC means the current only travels IN ONE direction.

This means the polarity of the signal never changes.

AC means the current alternates the direction so the polarity of the signal changes with time. 

Digital (when referring to signals) means the signal can only have one of two "states" or "values" (on or off). Whereas analog signals can have infinite different "states" or "values".

Here a few graphs explaining the differences: