Generally, you can either get a PhotoKey for 1 Day or Unlimited. Unlimited just means it is good for the whole year.
The only real difference is how long it is active for. They both still offer all your pictures in the park whether its on a ride or by a park photographer, but you get a digital copy that you can download to your phone via their app or to your computer via their website. This is good for social media sharing pictures.
If you want prints outs you get a discount on those.