The Public Test Build is the beta version of the Discord Client and the middle ground between the Stable and Canary clients. The Public Test Build (PTB) sometimes has features that are not available on Stable. The PTB is intended to be a way for users to help Discord test new features, so it can be expected that PTB is less stable than the main client. PTB tends to receive features later than the Canary build. If you find a bug on PTB, please report it on the Discord Testers Discord server.
The PTB build has the same icon as the Stable build.
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 the PTB that may be accessed by going to https://ptb.discordapp.com.