Mastodon and Lemmy are both built on top of the same protocol: ActivityPub. That said, they provide a substantially different user interface. Mastodon is focused on microblogging (basically, a Twitter clone), where you follow individual users and boost (retweet) or like posts. Lemmy is focused on communities (basically, a Reddit clone), where instead of subscribing to users, you subscribe to communities. There is some interoperability, but it kind of like a Spanish person trying to speak to a Portuguese person at this point.

Long story short, even though the goal is for all content to be accessible regardless of which platform you are using, there is a lot of content that can only be understood if you are using the correct one.

And they actually federate with each other! Trying to figure out how to federate prolewiki's mastodon with lemmygrad 🤔

But lemmy is more like reddit (link aggregator) and mastodon is more like twitter (micro-blogging platform). Though we get 2000 characters on mastodon to write a "toot" with, which is a lot and almost as much as you can have on Lemmy. The lines keep getting blurred and I think at this point people associate more with the UI/UX than the tech.

Although the main difference I think is on lemmy you follow communities, and on mastodon you follow individual accounts.