oAuth is an open protocol to allow secure API authentication in a simple and standard method from desktop and web applications. oAuth is not not a new concept. It is similar to your Google AuthSub, AOL OpenAuth, Yahoo BBAuth, Upcoming API, Flickr API, Amazon Web Services API but more open and not proprietary.
How is it more safe? You have a guest at home. You feel good to have a guest at home and you want him to enjoy his stay. Since your and your guests timings are not matching you like to give him key of your home. So he can manage when you are not around. But this key is a special key and not your regular (master key) key. With this key your guest can enter your home and kitchen but not your bedroom. oAuth works on similar principles.