Passing by value/reference?

Okay, so I don't really get this and the way my book explains it just makes it worse.

Passing by value is used if a parameter's data flow is:
A: one-way, into the function.
B: one-way, out of the function.
C: two-way, into and out of the function.
D: A and B
E: B and C

Passing by reference is used if a parameter's data flow is:
A: one-way, into the function.
B: one-way, out of the function.
C: two-way, into and out of the function.
D: A and B
E: B and C

Can someone please explain these to me and how you know the difference?
And while we are at it whats the difference between int& and int?

To put it simply, when passing by value, the value of the variable you put in is copied over to the variable in the argument list of the function.
For a reference, the variables in the argument list of the function becomes aliases to the real variable you pass in.
int& is a reference to an int.

To see this in action, make a function that modifies its arguments. Eg:

Code:

void foo(int & n) { n = 10; }

Do that with and without the reference and you will soon realize what happens.
Hint: print out the value of the variable you call the function with.

Practically speaking, when you additionally consider things for which const-references are suited to, the answer to the second one is actually F (where F is some combination of the others)

So with your last question, are you saying that you don't actually know what a reference is?