Sowbugs and pillbugs are not insects. They are crustaceans and as such are more closely related to crabs, shrimp, etc.
Sowbugs are grayish in color and prefer quite damp habitats. Pillbugs are bluishblack. Unlike sowbugs, they can role up into a ball like an armadillo when disturbed. They appear to tolerate dryer conditions than sowbugs. Both can measure from ¼ to ⅜ inch in length when full-grown. Sowbugs are the major food prey of the common Steatoda spiders.
Sowbugs and pillbugs are often considered as pests by gardeners because they are sometimes associated with gardens, found under boards in gardens, and in a variety of damp places. In many cases, they are found in rotting damp wood such as stumps or in cedar shakes where there is a moisture problem.
They are generally considered as beneficial creatures, due to their role in recycling nutrients.