Array Formulas differ to ordinary formulas. Just two things I'll mention quickly is how they are entered and the braces you see on either end.
1. When you enter an array formula, first select your range with the mouse, then enter the formula by pushing Ctrl, Shift and Enter at the same time, (don't just push Enter)
2. When you enter an array formla, two braces will appear at either end like this.
Don't try to enter the braces yourself, Excel will do it automatically.