It probably depends on which client you use but it is kind of a forum tool like reddit (striking similarities).

As someone who has specific needs in terms of ui and also designs UIs, I can try showing the differences.

Lemmy and reddit have communities (or subreddits, forums if you will) - the content is rather long form. I don’t know of any limit. You have forum like headlines and body texts. You can put in images, videos, links but the vast majority of posts is text.

Mastodon and Twitter are more short form (character limit) and they work more with threads, like answering yourself or others and so on. You can do this on the redditlikes but you cant do it the other way round. They’re also called microblogging platforms as you‘re limited in the length of your „blog“.

Then there is Instagram and pixelfed (havent tried the latter yet) which are focused on pictures and videos, less on text. I believe you can post as long a text as you like but without a striking image, nobody will read it.

And last (maybe least) is facebook: a wild mix of all of the above. I don’t think there‘s a character limit, you can post text, video, audio, whatever. Most importantly imo is that you join groups as these open you up to other people than your friends. There you again find a wild mix of post types. Facebook is notorious for the low quality content that some people post, like childish pictures of badly animated birds which are supposed to convey some kind of message. Those are signs that the less tech literate and sometimes less culturally knowledgeable are on the platform (imho).

Just to finish this off: mastodon, lemmy and pixelfed are part of the fediverse. This means they are federated and are not (and can never) be controlled by a single entity. Also, they don’t use an algorithm. They do not check what you like and throw stuff at you it thinks you like. These algorithms are like heroin to a lot of people but for some reason they‘re not outlawed yet.

Have a good one.