TL;DR: the "Canonical" version of Kubernetes has been customized by Canonical, the developers of Ubuntu, based upon their experiences of running Kubernetes. "Kubernetes Core" on the other hand is the standard, official, distribution you would find on Kubernetes.io. If you are looking to understand Kubernetes on Ubuntu then I would recommend the Canonical version, if you are wanting a standardised version then go for Kubernetes Core.
Spell is a function of Conjure-Up which is a product created by Canonical to be able to provide "Turn Key" installations of popular Open Source technical stacks such as Kubernetes and Open Stack. These stacks are typically non-trivial to install, so Conjure Up combines the installation of the software and configuration based upon guided best practice through a GUI and CLI interface.
conjure-up lets you summon up a big-software stack as a “spell” — a model of the stack, combined with extra know-how to get you from an installed stack to a fully usable one. Start using your big software instead of learning how to deploy it.
Conjure Up is capable of deploying to Ubuntu, MacOSX and popular public clouds.
At the time of writing the following spells are available:
bigdata incorporating: Hadoop, Spark and Kafka