This is the most common type of switch, and is used where a light is controlled from a single switch (although you will often see a two way switch used, with one terminal left unused).
There are two terminals in a one way light switch. When the switch is on, both terminals are connected together.
Typically, these terminals will be marked COM and L1 (sometimes L1 and L2). Although, technically, it doesn't matter which way round you connect the wires, it is best to stick to convention and connect the permanent live (from the supply) to COM and the switched live (to the lamp) to L1.