Lemmy

If you mean lemmy as the name of the website then yes. All these instances(or servers) create the whole network which lemmy is trying to build. For example you signed up with lemmy.ml, an instance that was created and run by the lemmy devs. This instance has communities(like subreddits) where people can post things. You can sub to these communities and they'll appear in your feed. But other instances like beehaw also have their own communities where people can post, and you can subscribe to those communities too(using your acc here on lemmy.ml, without signing up on beehaw)

lemmy and beehaw have their own list of members and their own admins with their own rules of whats acceptable and not.

When you become more comfortable you'll realize lemmy can also interact with other fediverse software( or websites) like mastodon but for now this is more than enough.

Instance is a server which is what is hosted on lemmy, beehaw, perthchat etc. When you register, you create an account on that server only.

Lemmy is the application that runs on these servers. Other examples of applications that could run instead are Mastodon, Friendica.

The Fediverse is the interconnected network of many servers and instances. You can post on a server that you aren't registered on, as long as they are connected (federated). If you consider all Lemmy sites that interconnect (e.g. Feddit, Lemmy, Perthchat, Beehaw) together as one "website" (a more accurate term would be social network), then from that perspective yes each server acts a portion of that "website".

The fun part is that Mastodon users and users of other Fediverse applications are also compatible, though there are some variations in terminology and methods that are suited for the applications use-case (tooting, crossposting, hashtags, etc.)

From a user's point of view it's just like email, you create your account on one server, you login with any app that supports that standard, you can interact with people from any other server, yet each server has a different owner and has it's own address.

it's like e-mail group but on steroids. Everyone can open his account where he likes. (E-Mail-Provider ~ Instance-Provider) but we can all communicate see each others posts interact in various ways and even get kinda different interfaces like Lemmy, Mastodon, Friendica, and many more.