This is a Banana Slug.  They are the second largest species of terrestrial slug in the world.  Banana Slugs can be a s long as 9.8 inches (25 cm)!  They are detritivores, or decomposers.  They basically eat things like leaves, animal droppings, and dead plant material.  They recycle this material into soil.

Banana Slugs usually have yellow bodies with brown spots. Some Banana Slugs can be found with green, brown, or white bodies. The color of Banana Slugs can change accordingly with their diet and the amount of moisture in their environment. The bodies of Banana Slugs have a muscular foot for locomotion.

Slugs are gastropods.  They are related to snails and mussels.  They live in oceans, lakes, rivers, and on land.  Their slime trail helps them get around and often times helps protect them from predators.