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.