To everyone joining right now:

Welcome to the fediverse. With your account on mastodon.online you get access to people from lots of other Mastodon servers (in fact, this message you're reading originates from mastodon.social!). You can follow people from these other servers and you can even move your account (incl. your followers) to another server. Beyond that server stuff the UI should be familiar enough, there's a lot of mobile apps.

Enjoy!