What’s the difference between the cover photo and a profile picture?

Your profile picture is the picture that friends see next to your name everywhere on the site. This is how people recognize you. Your cover is the large image at the top of your timeline. This is your chance to feature a unique image that represents who you are or what you care about. 

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Note: Like your profile pictures, cover images are public, which means anyone will be able to see them.
Source:http://www.facebook.com