Here's the official definition:
A
rule is a function if

Check out this guy:
Let's graph those points:
R
is NOT a function since one element in the
domain (1)
goes to two elements in the range
(2 and 4.) Or, you can just say that R is not a function since 1 goes to 2 AND 1 goes to 4. So, it fails the vertical line test! 
YOUR TURN:
Is  
a function? 
* put the letters along the bottom since they are the domain guys!
Is this a function?
* now the numbers go along the bottom!