That’s a fairly big question.

Lemmy works a lot like Reddit in terms of form factor. “subreddits” are “communities.”

A major difference is the federated nature of the platform. Reddit is one website. Lemmy, and in fact the entire Fediverse, are many individual websites. You joined lemmy.world. I joined sh.itjust.works. These two websites share data via a protocol called ActivityPub. From the user standpoint, don’t worry about it too much, you can think of it a bit like email in that gmail.com and icloud.com speak the same email protocol so they can send messages to each other.

A username will be formatted as @bluedeepeye@lemmy.world and a community name as !asklemmy@lemmy.world

On Reddit, there was only one namespace. If a subreddit name is taken…well shit. So if you wanted r/example but someone else has it, you’d have to do r/actualexample or r/realexample or r/examplemasterrace. On the Fediverse, each instance has its own namespace, so you’ll find several Ask Lemmy’s, like 40 Linux communities, etc. The pot community is even called trees.

Oh, and that ActivityPub protocol? It allows some kinda crazy shenanigans. Lemmy, mBin and Piefed all three share Reddit’s form factor. Mastodon is similar to Twitter. Pixelfed is like Instagram. Peertube is like Youtube. Loops is like Tiktok. All use ActivityPub, so you can do goofy shit like comment on a Peertube video from a Mastodon account. Imagine tweeting a tiktok. Not…reposting a link to a tiktok video on twitter…tweeting a tiktok. It’s bizarrely interoperable like that.

I want to add an additional point or two; unlike Reddit, people generally don’t care about karma (total up votes aren’t really counted and some instances don’t even do downvotes, etc) so even if we were mainstream you won’t see much karma farming. People post genuinely, although it might just be a dudes memes on their phone, etc.

Another is regarding the culture, as you may still get banned from specific instances. For instance, there are tankies (authoritarian communists) in places we mostly defederated from that ban you if you criticize the Chinese government or even North Korea (!). World, a typical go to for Redditors, tends to be center left but had their own controversial moments. Personally I don’t mind their defederation choices except I have to use another instance to access pro-piracy stuff (which div0 folks have the best of).

A couple of major differences from Reddit are:

1. Karma does not matter.
2. Since not all instances recognise each other, what you see will vary wildly depending on your home instance.

Also there is some self-sorting based on preferences and politics. Your instance (lemmy.world), for example, tolerates transphobes and zionists, and so attracts transphobes and zionists. (It may also gets blocked by other instances who don’t like this, so you won’t be able to see content there even if you personally are fine. This is one of the problems with lemmy.)

Be prepared to make multiple accounts.

That is the worst thing about the fediverse and why normies hate it. People want 1 login. This is why gmail is so popular. Many sites with single logins.