Lemmy is more of a Reddit alternative, where you have the focus on communities/categories (similar to "subreddits") to which you can subscribe to, while Mastodon is more a Twitter alternative, focusing on users to follow.
Kbin is actually slightly special.
It is also running on the Fediverse network, but can communicate with multiple other services. Meaning, it can show you content from all Lemmy instances (called Threads) and content from Mastodon (called Microblog). You can see this thread there as well.
In addition, it supports even more of the so-called "ActivityPub" services, like Pleroma or Peertube.
But Kbin has it's small hiccups here and there - it feels less reliable/stable. I had the best experience so far by using a Lemmy instance for Lemmy and a Mastodon instance for Mastodon.
By using Kbin, you have access to both worlds, Lemmy and Mastodon, in addition to other services.