Overdrive and distortion are terms that are (often wrongly) used interchangeably. Granted, they're essentially versions of the same effect - with distortion being overdrive's extreme big brother - but they provide vastly different playing experiences depending on what you go for.
The main difference is that, while a distortion pedal provides the same (or a similar) amount of distortion at any volume, an overdrive pedal reacts more like an old school tube amp - with the amount of gain and grit being dictated not only by the amount of gain you choose, but also by the input and output volume.