if... else... & elseif

Hi, since learning PHP i've come across this and i'm a little confused. Was wondering if anyone could shed some light on it.

I'm more than familiar with IF statements from C++. However i'm confused as to why you might use an ELSEIF...

why does it not work like this (below)?

IF(This is true)
{
do this
}

IF(this is true)
{
do this
}

IF(this is true)
{
do this
}

else
{
go home, you're drunk
}

I don't under stand why if you make the first IF statement every other IF statement has to be an ELSEIF.

Many thanks,

Royal1664