Fediverse is a social media, but it's not a typical one. Fediverse is a network of social media and most importantly it is decentralized. So all those most popular social media like Facebook, Twitter and Instagram are centered right. They are owned and maintained by a central corporation. There are pros and cons for both decentralized and centralized ones. If you are someone who doesn't like to the feeling of a central company being owner of your data and information shared, then you should consider moving into something more decentralized.
If to say a little bit about the history of fediverse, actually fediverse existed since 2008 but then there was only one instance named identi.ca then it wasn't called fediverse. And it wasn't decentralized neither. But everything changed in 2012 identi.ca got split into two instances called gnu social and pump.io, and apparently the word fediverse was originated during this time too. And until like 2016 GNU social was the flagship fediverse instance, but in changed 2016 when a new system called Mastodon was made with Ruby. And then in 2017 another server system called Pleroma was made. And even today Mastodon is the most famous fediverse software used by like more 90% of fediverse users. Now fediverse is massive with thousands of instances with thousands and tens of thousands of active users! And at the end of 2017 one million users were counted but a fraction of them were not active. But now it's pretty sure the user base is much more bigger. Specially around in 2019 thousands of new Indian users flooded into fediverse after twitter had some issues in India.
Actually fediverse is open for anyone. But you mostly it is full of techies, activists, journalists, artists, free software community, open source community and so on. If you love computers it's pretty sure you will find a lot of amazing people there.
How to join?
It's easy you can click here to see the available server systems that you can join. So the most famous one is mastodon. If you like instagram style ones then Pixelfed is for you. If you like something like facebook then may be Frienica or Diaspora will be good. Or if you like something like Youtube but decentralized then Peertube is for you.
So those are just server systems okay, in server systems there are instances. You have two options, you can join an already existing instance or you can host one by yourself. If you don't know and don't have the money to do that you can just join an already existing instance, it's totally fine. If you want to use mastodon then you should join a mastodon instance. If you want to join Peertube then you should join a peertube instance.
To get started you can go to this website https://jointhefedi.com/ then click 'join a server'. Then it will show many available servers. So as you are new to this I would recommend going for something like Mstdn.social. And then you can simply follow the instructions there to join the network. So the wonder of this is, it doesn't matter what server you are going to join. Because after all, those servers are connected. So you can talk and interact with a pleroma user with your mastodon account. Fediverse is a massive network of thousands of instances with thousands of users.
If you do have a question or have trouble signing up for fediverse do write me a mail.