Basic idea behind it
OAuth basically is a token-based authentication method without revealing the user password.
Why we need it
We visit lot of websites daily. Some of them ask us to register on that website to get more information. And normally the user does not register and left the site, because it is boring for the user to fill up registration forms etc.
On the other side by perspective of company it is very important to keep the customer’s details. If you don't have customer details then how you can tell them about your products.
Think you are an end user and you visit a website. You like there some products say laptops etc. And then you want to do some more research about that product. Here you left the website. And the website does not have any way to communicate with you as you are not registered with website
How it works
Here OAuth comes as life saver; It allows an end user to share his/her information by using by third-party services, such as Facebook, Twitter, LinkedIn etc.
OAuth acts as an intercessor on behalf of the end user, it provides the access to the account information with an access token. User can revoke the access anytime to stop to share the information to that website.