The new handoff feature by Apple brings proximity awareness to your Apple devices. If you are composing a Mail on your iPhone you Mac will recognize that and will notify you with an icon near the dock, you can then just start composing the same mail on your Mac where you left off on your iPhone. The same works with other apps like Messages and Document editing apps.
Not only this you can also receive and make calls from your Mac without using your iPhone. When you receive a call on your iPhone, you will also be notified on your Mac with the caller ID from where you can answer the call or reject it. You can also reply to your iMessages and text messages from Mac via Handoff feature which makes your user experience more productive.
There is another feature called Instant Hotspot through which your Mac will automatically recognize the data connection on your iPhone and start using it. You just need to enable the Internet Tethering on your iPhone and Instant Hotspot will do the rest for you. Cool isn't it??