Just a couple of months since the availability of Fedora 24, the Fedora Project has made the first and only alpha of Fedora 25 available. It's relatively easy for anyone to grab a copy of the ISO and help test and report bugs. Once everything that was reported has been fixed, a beta snapshot, similar to the final release, will be made available for download.
The main feature in Fedora 25 is that it will be shipping with the Wayland display server which has been in the works for over half a decade. By shipping with the new display server, it will provide impetus in the near future for other leading distributions to keep up and follow suit. The other main features of the release are that it'll come with GNOME 3.22 (currently 3.21 developer builds until 3.22 is out), and all the improvements that offers, and Linux Kernel 4.8.
In the email announcing the availability of the alpha build, it says:
“The Alpha release contains all the features of Fedora 25's editions in a form that anyone can help test. This testing, guided by the Fedora QA team, helps us target and identify bugs. When these bugs are fixed, we make a Beta release available. A Beta release is code-complete and bears a very strong resemblance to the third and final release. The final release of Fedora 25 is expected in November. If you take the time to download and try out the Alpha, you can check and make sure the things that are important to YOU are working. Every bug you find and report doesn't just help you, it improves the experience of millions of Fedora users worldwide! Together, we can make Fedora rock-solid. We have a culture of coordinating new features and pushing fixes upstream as much as we can, and your feedback improves not only Fedora, but Linux and Free software as a whole."
The Fedora Project is not the best at setting accurate release dates, with extreme delays occurring from time to time. The Fedora 25 cycle has already experienced one set back, the alpha should have gone online back on August 23. As a consequence of the delays, it's hard to put an accurate date on the beta and final releases, but as it stands now, the beta is due October 11, and the final release is due November 15.
Source: Fedora 25 schedule