How does facebook or any site know if a password entered is correct or incorrect?

[Java]

Assuming you know some basic programming you should understand this...

First you have a password, you type it in. It checks on a server if your "String" is the same as the stored "String" if it is proceed! If not don't. Now all of these passwords are stored on a server / MySQL database. (Basically a giant storage of anything) (Like a huge Google drive or SkyDrive). (Look it up it is fascinating stuff) Anyway that is how it checks it and stores the password. and for the "Tricking it" that is why the code is protected, that is why on some sites you can "trick it" and effectively "hack" into the MySQL database. That is how some modern hacking works. So technically yes. Someone could hack your email and get into it. That is why you use giant email corporations because it is almost guaranteed that no one will find a way to "trick" the servers. Hope I helped!