There are about 30 species of slug in the UK of which only those four can be considered pests. It is estimated that the average garden has about 20,000 individual slugs and agricultural land will contain 250,000 per acre. They mostly live underground and come out at night, using their slime trails to find their way back home.
Like snails, slugs have two long tentacles which they use to smell and dimly see, plus two shorter tentacles below, with which they feel and taste stuff. In the photo above you can also make out the thicker 'mantle', the smoother bit just behind the head. When they are alarmed they pull their tentacles in safely under the mantle.
Defra slug identification
Identifying British slugs