Bring me up to speed with 6 seasons on Pretty Little Liars, please!

[jiggeryjaggerywoo]

I finished season 1 earlier. I enjoyed it but not enough to sit through 6 more seasons of 25 episodes each.
So, I've decided to just watch the final 1.5hr episode. I'm about 30mins in and now need to wait for the DCs to bed before I can continue

So, I need spoilers and explainers. Who was A? What happened to Mona? When did Alison come back and where had she been? Who is the "mum" that Spencer is visiting in prison? How is Caleb so utterly utterly gorgeous?
Spill the beans please!

Oh mate, that’s a big ask. I only made it to season 4 and that’s still a lot to explain!! Are there any recaps on YouTube?

Ha thanks, I know it's a big ask! I think I need to look up the individual characters on fan pages to find out their stories. 25 episodes per series is just madness and I don't have the patience for it unfortunately...