What is Oauth Authentication. Is it safe..?

Now a days Internet is very comman in our day to day life . We even cannot think our life a second without Internet. Our life is surrounded by many websites and mobile applications . Some time you guys have seen some websites in which you have to login but you don't have account on that particular website so either you have to create that account by clicking Register or Signup button which takes longer process as after clicking you have to enter many details like your Name, Email, Mobile no. etc. and after that sometimes the sites will give you the verification code or link to click on it to verify. But its a long process you must have also seen an option of Login with Facebook or Google or with you Twitter account this , other way of logging into the websites with actually registering it is know as OAUTH AUTHENTICATION. Is OAUTH AUTHENTICATION safe..? Now the question arises is it safe to Login with...