I’m an electrician, but I’ll try to keep this simple. Let’s just pretend that your typical 9 volt battery generated AC instead of DC. This battery has your typical two terminals in which one is the positive and one is the negative, so with the chemical reactions within the battery cells that generate DC the polarity of the positive and negative terminals stay the same.
Now, getting back to the hypothetical battery which will generate AC, the polarity of each of those positive and negative terminals would constantly reverse. Depending upon the frequency of the AC waves (which could be sine, square, sawtooth, etc) the polarity of each of those terminals will always change at a certain time. The typical American house circuit has a 120/240 volt single phase system which operates at 60 hz. 60 hertz would mean that the polarity of those two battery terminals would switch at a rate of 60 times per second.
As far as uses for each go is pretty variable. AC is better suited for power transmission because it can be transformed easier and thus the voltage can be stepped up. The higher the voltage the easier it is for it to travel longer distances due to voltage drops by the resistance within the conductor used itself. AC transforms better than DC because the polarity of any inducted current reverses itself, and because DC doesn’t change polarities it can only be inducted once, unless you pulse it. Electric fences and ignition coils are examples of using transformers to induct and transform voltage using pulsated DC.
DC is good for many electronics uses. DC is also good for loads which depend upon strong magnetic fields to function properly such as electric magnets, coils, certain motors, etc. Well I won’t drown you here but I hope that helped. I’m not a great teacher.