Canary is Discord's alpha testing program. Due to Canary being a testing program, it is usually less stable than the normal build, but usually gets features earlier than the PTB or Stable clients. The Canary Build's purpose is to allow users to help Discord test new features. Bugs on the Canary build are supposed to be reported on the Discord Testers server.
You can find update notes on Discord Canary specific to each version at the links below.
The Android and iOS apps have independent beta testing programs. If you are interested in participating in these beta programs, join the Discord Testers server and read the frequently asked questions.
There is a web version of Discord Canary that may be accessed by going to https://canary.discordapp.com.