The update is available for iPhone, iPad and iPod Touch via Software Update on your device or via the iOS Dev Center. It weighs in at 305MB for the iPhone 5s.
There's many changes in iOS 8. Here's a few of the changes we've posted on already: Continuity, Handoff, AirDrop, Instant Hotspot, Health, iCloud Drive, Family Sharing, Photos, Camera, Mail, Safari, Messages, QuickType, Third-Party Keyboards, Interactive Notifications, Spotlight, and Contact Shortcuts.
We'll be posting on what's new in the fourth beta shortly. If you spot anything please let us know in the comments. Follow iClarified on Twitter, Facebook, Google+, or RSS for updates.