We have Lemmy.world, Lemmy.ml, beehaw.org, I think even yiffit.net. If I log in using wefwef it asks me where I want to login. And when I make a new account using another instance, it says ”… Lemmy is federated, so you can interact with everything on lemmy.ml even if you’re registered on a different instance.“ So if I’m registered on lemmy.world, how can I interact with lemmy.ml?
The comparisons to email are a good analogy. Your email is hosted on a service (e.g. @gmail.com or @hotmail.com) each has its own rules and setup. They also run independently. However, you can happily email someone at a different host. The hosts sort it out in the background. It’s completely transparent.
With Lemmy, the instances are the equivalent of email hosts. You might use your work email service, or use a free one, or even pay to host your own. They will all play happily.
De-federation even has a good comparison. If an email server is sending masses of spam emails, the other hosts can decide to not accept emails from them. They get cut off. The decision is made on a host by host basis however. This contains power tripping to individual instances.
From a user’s point of view, there are 2 main things to note. Local and All. Local only shows you posts on the instance you are logged into. This can be useful, of the instance is specialized, and you want to focus on that specialty, but not be limited to just 1 channel. This could be something like programming.dev, furries, or a NSFW instance.
All does what it says. It shows all posts on both the instance you are logged onto, and any your instance is federated with (basically everyone, other than those your admin has blocked).
As for which instance to choose? 3 main things matter, Focus, Ideology, and Stability. Focus is for if you have a specific interest. You are aren’t limited to it, but a programmer basing themselves in a programming instance makes local channels more useful. Ditto for someone posting NSFW content.
Ideology is related, but slightly different. Different instances have different mentalities. E.g. Lemmy.world is the “default” while beehaw is more focused on providing a safe space over open access. Lemmygrad is full on tanky communist, and de-federated from Lemmy.world. Each has pros and cons, producing unique internal cultures.
Stability is likely the most important right now. Instances are run by individuals right now. This means they can only scale so far. One of federations goals is to spread the load. By using a smaller instance, you can gain a lot of stability, while losing little to nothing in access. Lemmy isn’t one thing supreme, it is a legion under 1 banner.
Hope that helps.