Of note, Microsoft will not roll out the preview to everyone, at least not to start. "Preview members who have historically submitted the most feedback" will be the first to receive the update, with the ultimate goal "to start with a group, get feedback, then roll out to larger groups for additional feedback over time."
If you do choose to opt in, you will get regular updates up until the official release in November.
The November update will introduce a completely re-hauled user interface, new backward compatibility with over 100 Xbox 360 titles and smaller updates to the store and OneGuide. Additionally, there is a new 'Community' section which adds a ton of social features.
Source:
Microsoft