Highlighted Stories First: Which means the most highlight stories such as the posts which gets more likes comments by your friends will be on the top of the list. It means you won't miss any important updates by your friends. So if you choose Highlighted Stories First then every time you visit your facebook news feed all the top stories will be on the top of the page. This will be a great feature for all users who have mass number of friends so that that can choose so that they won't miss anything important.
Recent Stories First: If you choose the option to recent stories first all the recent stories updated by your friends will be top of the page according your friends updates. That means whenever your friends update something status, photos, comments or anything you will get notify instantly inside your news feed. It is same as before as you can say the old Facebook news feed. So, those who ever like to have their old Facebook style must choose this one.
So how can we select news feeds ?
Just go to your Facebook home page. You will able to see a Spot button below the status bar. click on the Spot and select your option either recent stories first or highlighted stories first.
You can learn more from this page