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.
Hi, one of the influx on new users here.
From a user interface point of view this is one thing it would be cool to iron out, if possible.
When I go to another server that's not my own, it would be nicer to just sign in there (using my existing account as a "single sign on" type thing) and being able to write my comment without having to come back to my own server first.
I know there are probably technical reasons for why that has not been implemented but I think it would be cool.
@confusedbunny @benx
It depends what you mean by "use".
You can follow and interact with these services from your Mastodon.
If you post a photo on Mastodon, it will be visible to people on PixelFed as if you had a PixelFed account.
If a PixelFed person posts a photo, it will be visible to people on Mastodon as if they had a Mastodon account.
There's not a huge difference there in functionality, so there's not really a need to have separate accounts.
In the case of BookWyrm, if you want to write a new book review yourself you would probably need to sign up on a BookWyrm server.
But if you just want to follow and comment on what BookWyrmers are posting, you can do that from Mastodon.
There's no central Fediverse account system because when you centralise stuff, it sooner or later gets bought out by Elon Musk type people.