How do you find the magnitude of a vector?

I am so confused about vectors. I have seen them listed as three numbers (1,5,3) or as two (3,4). I have read to find the magnitude of a vector you subtract (x1-x2) and I've read that you add (sqrt of u1^2 + u2^2 + u3^2) I am so confused. I know that magnitude is length, but I am so lost!