What is fediverse basically ? How does it work ?

a bunch of forums that talk to each other.
a decentralized collection of independently operated social media platforms that can choose to share information with each other.
- Whenever anything is "federated" the easiest comparison is email. Everyone can use different email servers and applications, but they can all still communicate with each other. "Email" is the shared protocol that structures how data is exchanged between servers and clients.
  The fediverse instead uses a protocol named "activitypub", but works similar. Just a bunch of servers that speak the same technical language which enables them to freely exchange information. But while email only supports single messages, activitypub supports more complex structures that developers for platforms like lemmy or mastodon can use to make actual applications with an interface that gives you an easy way to display and structure that data like as a forum or blogging platform.
  
  Whenever anything is “federated” the easiest comparison is email.
  Yes, but how many people know how email works? More to the point, how many people, who aren't looking for the answer on GitHub, know?
  
  Email is a terrible example that people need to stop using.

This guide from the people who run lemmy.ca is a good starting point.
- This one is a really nice link, thank you ☺️☺️☺️

Like email but in the style of platforms like reddit, twitter, YouTube, Facebook, etc. There is no one site to create an email yet you can send an email to anyone no matter who their provider is. The fediverse aims to do that but with other platforms.

I suppose you are on lemmy. Lemmy is kind of the reddit of the fediverse. Imagine that now everyone can create their own reddit server with blackjack and hookers. That is the server you are on, someone decided to create a server for the world. But now I am on another server with different owners, rules and mods (basically blackjack and different hookers). I can interact with you on your server and you can interact with me on my server because our servers are federated.
Now imagine that you can do this for every social media. Reddit, X(itter), Instagram, YouTube, Tiktok, WhatsApp. Everything is available in the fediverse (with more or less issues depending on what you are looking for).
- Great 🤓🤓.... So you've replied to me from mastodon.social ??
  
  Looks like they are on feddit.org to me.
  Your client will usually show @
  <server>
  after the username. If not, check your client settings if you’d like to enable it to display that.
  
  @docker @oakward here i am from infosec.exchange
  
  @docker @oakward@feddit.org I did not answer you from mastodon. I was answering you from another instance of lemmy (feddit.org). But now I am writing to you from mastodon (mstdn.social). I don't find this maston-lemmy connection very useful though.

The fediverse is a diverse social network that is distributed across multiple platforms.
Going from the basis to the specific services:
The ActivityPub protocol defines a protocol - data structures and forms of data transfers
The Fediverse is a collection of different social networks and services hosting these social networks that can and do connect and communicate with each other
Various social platform software like Lemmy, Pixelfed, Mastodon implement ("speak") ActivityPub and are part of the Fediverse
Anyone willing can run (host) their own instance of these, by downloading and running them
The platforms have difference interfaces and capabilities, but in one form or another, users can connect and use other platforms despite them being "different"
For example:
Lemmy is a social platform like Reddit, but anyone can host their own instance. Users can make accounts and create communities on different instances. Users can then follow and interact with communities from other instances.
For example, my account is on feddit.org, but I am posting this comment to !asklemmy@lemmy.world. Because I follow the community, feddit.org fetches and copies the data from that community, and any activity from either side gets communicated to the other. As a result, you on lemmy.world and me on feddit.org can both see and interact with the same community.
Mastodon is a social platform like Twitter. Someone may have their account on mastodon.social, which is one such instance running Mastodon. Despite being a different use-case and interface, they can follow, read, and post to Lemmy communities.
I tried replying to this comment from mastodon.social. I was able to view this post and this comment. I don't know if the comment will show up or not; maybe lemmy.world blocks mastodon.social comments.
The Fediverse is the collection of compatible services and networks that can speak and connect to each other.
- Great 🤓🤓 So you replied to me from mastodon.social ?
  
  No, I replied from feddit.org.
  On lemmy you can see where the account is from
  and you should be able to see Kissaki@feddit.org.
  I tried to reply to my long comment from mastodon.social, but it didn't show up. I assume lemmy.world blocks replies from mastodon.social.
  Given the diverse nature, instances can control to what degree they connect to other instances within the Fediverse.
  One instance may block another because of spam or extremist or illegal accounts or content. Or they may allow interaction but only for those explicitly seeking it out instead of showing it within their own interface. etc
  
  On Mastodon it looks like this:
  My reply (second screenshot) did not show up here on Lemmy. I assume because lemmy.world blocks replies from mastodon.social.

Various servers using various software that can communicate with each other.

Hi!
Since you're on Lemmy, I'll explain the part of the fediverse that I know best. The best way I can articulate it based on what I know is this... it's a bunch of different sites, using the same software, known as Lemmy. Each site using the Lemmy software is called an instance, and these instances are able to correspond in a way that is pretty seamless as if it's all one big site! Although you never actually leave your native instance, it doesn't feel that way because when you post to an instance that isn't your native one, it's like sending a request to that instance to push your post forward. In no time, your post appears on the remote instance!
One of the best things about this is each instance is independently operated, so you don't have to worry about a rogue admin making unfavourable changes. For instance (hehe) I'm on the lemm.ee instance, but I'm posting to Ask Lemmy@lemmy.world via this page! lemm.ee/post/62441272