The esiest way to zoom in and out on a Google Map, is to use the Mouse Wheel (up for zooming in, and down to zoom out).
Sometimes this isn't possible (by default mouse wheel zoom is disabled for API usage, and it's hard to use a mouse wheel when you're on a laptop with no mouse!)
In this case you can double click with the left button to zoom in (I'm sure everybody knew this) - but you can also zoom out by double clicking the right mouse button.
(And if you are using a Mac Book, then tough luck as Apple are still insisting that a right mouse button is superfluous!!!) (...waits for flames from Mac Fan Boys...)