Nightly packages contain the newest features of the toolkit. Released a few times per week, they are not intended for production ready code, as features may break or are under constant changes. They also have a companion symbol package (snupkg) for easier debugging.
These builds are normally built from the alpha branch, or from a branch with the alpha- prefix.
The canary packages are the next phase of development. Typically, canary packages are released once per month. Although these builds are considered to be more stable than the nightly packages, they are again not intended for production ready code, as features may break. Canary packages are there for developers to test & evaluate new features for the final release. They also have a companion symbol package (snupkg) for easier debugging.
Stable packages are normally released once or twice per year, and are intended for production ready code. These packages are supported for 12 months from the date of initial release, i.e. from November until the following year, excluding any patches released between them.
For more information about supported frameworks, please check out this article.