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

[Izzy]

I like to think how things work and I have done programming for 3 years in my hs but the stuff is so basic. Like even the advanced stuff we did were easy so I never really 'leanred' anything after the first yr and ive been wondering how it detects the password. Like it has a bunch of users right? where is all of this kind of information stored? What if someone tricked it into thinking a correct password was entered even though there was nothing in the field? Wouldn't that mean that with your email they could just get into your account? Is that even possible or is there some kind of strict link between the user/email and password?