Login account implementation with other networks account

I'm currently consider to implement a login system by using php+mysql, it haven't got any major problem.

However, I've notice that, more and more site currently not only use their own login system (actually, some of them are remove their own login system), but use different social networks login system (twitter, facebook, google...just like stackoverflow).

Can someone explain what is the pros and cons of these? And if using other login, are they grab the data from the facebook (for example with facebook login) or just use it as login, to prove someone who are a real human...?

(It is great if someone 开发者_StackOverflow社区can provide some php example :) )

OpenID

stackoverflow.com uses OpenID. Jeff Atwood(Author stackoverflow) even has an article explaining why stackoverflow.com uses OpenID. But to me(also Jeff Atwood) the most important properties of these (social) logins are:

• I don't have to store(probably insecure) the passwords anymore and that the user will have less identities(passwords) to remember.
• Even if the password get's exposed it only has to change the password in one/that spot instead of hundred spot or it has to be smart enough to not use the same password for every site, but then it will be insane difficult to remember the passwords.

Example

For an example i would advise you to read this answer from me about OpenID.

This is a complex question. It's cumbersome to create yet another account for your visitors, for them using Janrain Engage (that's the best IMO) is way easier. But then you are looking at trust issues, using a rather complex protocol etc. On the other hand, it's becoming really cumbersome to create yet another account so please don't unless you have a checkout process where you can more easily ask for a password -- under no circumstances ask for a username though unless you really, really must, the email address should always suffice.