They’re both forums that look and act similar to Reddit.
Fundamentally - they look different, are coded differently and have some jargon differences.
For example, the communities in Kbin are called “magazines”. A lot of folks still say “sub” as a holdover.
Yet, they’re both part of the Federation, so you can up/down vote, comment and subscribe to each other’s stuff from your home.
Federation is similar to having a group text with friends. It doesn’t matter if you have an iPhone and they have an android. It doesn’t matter if you’re on Verizon and they’re on ATT. The software is designed to communicate with each other.
Lemmy differs from Kbin in that it can be franchised into an independently owned and operated instances. So there’s multiple Lemmys like Lemmy.ml and Lemmy.world. Each instance can set its own rules and even chose which other instances/platforms it can federate without. More importantly, nobody owns all of it.