This slimy creature is a garden slug. Garden slugs come from Europe and they are famous for eating plants in our yards. Most of our native slugs do not do this since they generally prefer living in the forest. No such luck with garden slugs.
Many people try to get rid of these hungry invaders using different methods. One that was used a lot in the past was sprinkling salt on their bodies. This does kill a slug, but scientists believe that it also causes them to feel a lot pain- not a friendly way to rid your garden of these pests.
Instead, I rely on another method:
See the head near the middle of the photo? This is a garter snake. They do not harm people since they are small and are not poisonous. But, they like to eat slugs. I have a lot of them in my yard because I leave small piles of sticks, leaves, and other plants around the yard. Snakes like to use these piles as shelter, as do a lot of other small backyard wildlife. So, although the slugs still eat some of my plants, the snakes help to keep them in check and keep the damage to a minimum.