This calculator uses postfix notation also known as Reverse Polish Notation (RPN). The advantage of this method over traditional calculators is that you can compute even complicated terms without braces.
An example: Suppose you want calculate 5 * (3 + 4)
Press the following keys on the calculator: 5, enter, 3, enter. The 3 and 5 did go into the calculator's memory, the stack. Now you type 4 and press + to add. After this you just press * to multiply.
You can either operate this calculator with mouse clicks or you can use the keyboard. However some web-browsers have keyboard shortcuts which may conflict with the keys used by this calculator. Therefore you need to place the mouse over the text area in the lower right corner as this avoids the browsers keyboard shortcuts to take effect.