Hi @jacor
I am still a new bee so i am still figuring out hive. But this is how I understand things:

What is the difference between Hive and Peakd?

Hive.blog and PeakD are like skins for hive.They do the same thing (blogging).
But they have different features. For egs peaks allows you to post gifs directly in a post.

Are there more platforms like Hive and Peakd?

Sure. ecency (esteem) and dbuzz are a few ,you can find more at hivedapps.com

How does communities work and which communities are the best to join?

Communities are like blogging groups, they post on topics only of their interest.
As for which to join... It depends on what you like.