Okay, it's time to tell new users a secret about Mastodon.
Mastodon is (whispers) part of a much bigger network 😮
Mastodon servers use an open standard called ActivityPub to talk to each other. That's how you interact with people on other Mastodon servers.
But, dozens of other federated networks also use ActivityPub, so (this is the good bit!) you can follow people on these other networks from your Mastodon account!
For example:
PixelFed (https://pixelfed.org) is a photo sharing network, here's a random example account @Iancylkowski
PeerTube (https://joinpeertube.org) is a video sharing network, with a p2p system that allows videos to go viral even on small servers, here's a random account @craftykat
BookWyrm (https://joinbookwyrm.com) is a social reading site, an open alternative to Amazon's Goodreads, here's a random account @mouse
Together, Mastodon and these other services form... The Fediverse 
(whispers) Click on the link...
https://framatube.org/w/4294a720-f263-4ea4-9392-cf9cea4d5277
The link you just clicked on leads to a video on PeerTube, on a server called framatube.org.
BUT... if you copy and paste the same address of the video into the search box in Mastodon, and search for that link within Mastodon, it will appear to you as a Mastodon post 
You'll then be able to watch it (of course), but also you'll be able to comment on the video by replying to the post within Mastodon, like the video by favouriting it, and follow its creator by following the account that posted it.
This is the Fediverse in action, totally different services interacting through a common open standard.
It can be hard to get your head round this at first, but it's incredibly useful and once you get used to it simply how things should be.
@feditips Wow!
The more I learn, the more I feel like I'm starting to realize how vastly more connected and organic the fediverse is, and i'm loving it.
Posts that are mutually accessible between platforms of different specialties. Linked content sharing between servers so data doesn't have to be duplicated again and again. Not just the lack of effort to keep you locked down to just one platform, but actively encouraging new variety and connected communities.
☺️ 🤩
@aaly
Yes, indeed!
Also, because the Fediverse shares a userbase, it isn't about competition. When one platform's userbase increases, it also increases the audience for all the others.
Even tiny and experimental platforms can work on the Fediverse because their posts can be seen from the larger platforms, they don't need a lot of users in order to have a lot of reach. So, this can encourage experiments and innovation too, in a way that walled gardens don't.