Handoff is one of the features of the Continuity suite that debuted with iOS 8 that lets you pass information between apps on different devices using a Bluetooth connection. For example, you could start an email on your iPhone, then hand it off to your Mac and continue from there. Handoff works with a number of apps including Mail, Messages, Safari, Maps, Notes, Calendar, Contacts, Keynote, Pages, Numbers, Phone and Reminders, as well as some third party apps.
Handoff is a highly touted new feature of iOS 8 and OS X Yosemite that allows for seamless continuity between your devices. For example, if you start an email on your iPad, Handoff will allow you to pass the draft to your iMac and continue writing it there. Handoff will also work with Safari, Notes, Keynote, Numbers, Contacts, Calendar, Pages, Messages and Reminders, and it is open to developers as well, so it will soon work with third party apps.
If you've been having trouble setting up Handoff, you aren't alone; there has been a lot of confusion about it over the past several days. Here are a few potential reasons for your frustration: