Is a snake's skin smooth

Depends on the species! There are lots of species that have what are called "keeled" scales. This is when there's a ridge on each individual scale, creating a rough texture. Garter snakes and are an example. The Angolan python is an example of a snake with beaded scales, where each scale is exactly that - little round bead-shaped scales. These kinds of scales are very bumpy-feeling. Snake scales can also be soft and smooth, such as with boa constrictors. So, in short, depends on what kind of scale you're dealing with.