It's basically twitter/microblogging* with multiple hosts, and you can see posts from other hosts. So instead of just @andrewzah, my handle becomes @andrewzah@<host-I-chose>. You could visualize twitter as @andrewzah@twitter.com, but it's redundant as twitter is the only host. This is basically how email works, no?
The only other difference is federation, so you have two timelines instead of one: the entire** federated timeline, and the local timeline to your host. Plus the timeline of users you follow.
So unlike twitter, the user must decide what host they want to use. Or they can self-host an instance. I'm not sure if it's up to date, but https://instances.social has a wizard for this.
*: IIRC mastodon's default limit is 500 chars, and pleroma's is 2,000.
**: One exception: Hosts can block other hosts. So if you use i.e. mastodon.social, you won't see posts from users on blocked hosts unless you specifically follow them.