I have always thought that a potato bug = pill bug = sow bug, but apparently not! The pill bug is the one on the right. Both can be found in damp places and feed on decaying matter. In a garden, there is some evidence that they remove heavy metals from the soil, but if there are large colonies, there have been complaints that both will snack on young plants.
Through searching for information, I discovered a great resource for identifying bugs on the Pest Control Canada website. Obviously there are other useful sites if you Google “identifying bugs”, but this one is Canadian-based.