Mastodon is primarily a microblogging social media platform akin to Twitter. The other two are primarily multi-forum board akin to Reddit.

All three rely on the ActivityPub protocol, so there is some intercommunication between them (esp. between Lemmy and Kbin). That's why they're often referenced in the same breath. That, and most websites operating under these standards are not run for commercial profit.

Mastodon is like Twitter, Lemmy is similar to reddit, and Kbin combines both functionalities with different terminology. I'm sure someone more knowledgeable will chime in !

• Mastodon: Like Twitter. You can follow people, or #hashtags.

• Lemmy: Like Reddit. Subreddits on Lemmy are "communities". Different instances have different communities.

• Kbin: Both Lemmy and Mastodon combined. Subreddits on Kbin are called "magazines". Posts can be made to magazines directly, plus Mastodon posts with #hashtags will be automatically cross-posted to the appropriate community.

All of them are part of the "fediverse". Because of this, they can all talk to each other. You can follow Lemmy communities and Kbin magazines from Mastodon. Kbin allows you to follow Mastodon users from Kbin.

All that changes is how the content is displayed. Lemmy displays it like Reddit. Mastodon displays it like Twitter. Kbin has different tabs that let you switch between both.

Not a ton, they're both link aggregators, and the servers can all talk to each other (except for some that defederate, I think beehaw has defederated)

my advice is to find a community you like and want to stick around in and make sure you can post and read there, and don't worry about the rest

Simplified answer: They’re like email servers, and posts are like really advanced emails. You’ll see the same content on all 3 of these since all the content is automatically emailed between them (you don’t need an account on every instance like you don’t need multiple email accounts).
Where things get different is how the content is displayed to you.