- iCloud - WIll keep all your contacts, calendars, email and now your documents in sync across all your devices.
- Reminders and Notes - They are now separate Apps that sync via iCloud with their iOS counterparts.
- Messages - Replaces iChat and now lets you chat to friends via iMessage from your Mac.
- Notification Centre - Has come to the Mac to keep all your alerts in 1 place.
- Dictation - You can now dictate to your Mac in any App.
- Sharing Sheets - Now it's easier than ever to update Twitter and Facebook without having to open a browser.
- Airplay - You can now mirror your Mac's screen over Airplay with your Apple TV.
- Game Centre - Apple's famous Game Centre from it's iOS platform now let's you play games against your friends on the Mac.
- Safari - Now has a unified address and search bar and has introduced Tab View.
- Powernap - Your Mac can now download updates , refresh photo stream and other iCloud services while it's in sleep mode.
Here's Apple's walk through for you: