Wookiees are large hairy humanoids native to Kashyyyk. Wookiees rarely wear clothing and are incapable of speaking "Basic" (the common language of the Republic and Empire), but they are quite intelligent. The Wookiees were enslaved following the rise of the Empire. Many Wookies were skilled contractors, engineers and navigators. The Empire enslaved wookiees to serve as workers in various engineering projects.
It was noted in the book Death Star that enslaving Wookiees was unnecessary. In many cases, they would have worked on the projects of their own accord for the technical challenges.