Apple Introduces Mac OS X Yosemite - Here is everything you need to know!

Its a complete redesign. With completely new, flat UI. We have translucent sidebars, menu bars, and notification center. The new notification center has all your info like Calendar, Date, Stocks Info, reminders! You also get widgets as well. Like calculator, sports widget etc. You can drag and drop it in the main panel.

New spotlight is really powerful.
Its appears in the middle of the screen and can now search for documents, and gives a preview of it, and conversions etc.

iCloud Drive is a folder that you can find in Finder where you can drag and drop pictures, images, videos and it will be available in all the other Macs and on iOS devices as well. And yep its also available in Windows.

Mail Drop - Attach large documents. You can now send files up to 5GB as it uploads the attachment to iCloud server and sends the message through the your mailing service. Then on the receiving end it gives the users a link to download the file.

Safari has omni bar with favorites appearing as soon as you click the address bar. Tab views gives you bird eye view of tabs and you can scroll through the tabs smoothly by swiping gesture. Safari gives user sharing options as well and now alongside Twitter, Facebook, Flickr, you also have the ability to send it through mail.

Markup is another feature of mail which allows you to edit your images before sending them. For example you can put a circle around any object you want to emphasise. You can add speech bubble and text and much more!

Continuity:

Airdrop now works between iOS and Mac.

Handoff is basically a simple concept that uses proximity awareness. So for example if you are writing a mail on iPad, an icon appears on Mail App allowing you to continue right from the same spot. (VICE VERSA) This works with iWork applications as well.

SMS does not show up on other devices. However now it does. Now when you get SMS on your phone, it will show up on iPad, iPod Touch and Mac OS X. Also it works with Phone app. So now you can accept calls from your Mac and also you can make call from Mac as well. How cool is that!