K3s is designed to be a single binary of less than 40MB that completely implements the Kubernetes API. In order to achieve this, they removed a lot of extra drivers that didn't need to be part of the core and are easily replaced with add-ons.
K3s is a fully CNCF (Cloud Native Computing Foundation) certified Kubernetes offering. This means that you can write your YAML to operate against a regular "full-fat" Kubernetes and they'll also apply against a k3s cluster.
Due to its low resource requirements, it's possible to run a cluster on anything from 512MB of RAM machines upwards. This means that we can allow pods to run on the master, as well as nodes.
And of course, because it's a tiny binary, it means we can install it in a fraction of the time it takes to launch a regular Kubernetes cluster! We generally achieve sub-two minutes to launch a k3s cluster with a handful of nodes, meaning you can be deploying apps to learn/test at the drop of a hat.
Both its reputation and adopton is growing rapidly too, with over 17k Github stars since its launch in early 2019, whilst it was recently crowned the number 1 new developer tool of 2019 by Stackshare.