Slugs have a bad reputation among the general public : they are synonyms of pests, eating our salads and other vegetables in our gardens. But the things are a little more complicated as you can think : if some species are indeed damaging for the cultivated land, other prefer, sometimes exclusively, decayed mushrooms or dead plants.
Such "friendly" slugs are the well known Tiger Slug (Limax maximus), the Forest Arion (Arion silvaticus) and Worm Slug (Boettgerilla pallens).
But it is right to say that the Garden Slug (Arion hortensis), Spanish Slug (A. vulgaris) and Netted Field Slug (Deroceras reticulatum) are fully pests indeed !
The Longneck Field Slug (Deroceras invadens), Large Black Slug (Arion ater) and Large Red Slug (A. rufus) are intermediates : it depends on what they find first to eat !
How can you identify all these species ? It is not really easy with a minimal experience, differences between some species being slight, with a high individual variability ratio.
If you want to take up such a challenge, consider especially these features : location of the hole used to breathe (the pneumostome), size of the mantle proportionally to the size of the whole body, presence or absence of a crest on the tail, colour (spots, bands) of the body (including around the pneumostome), of the bottom of the foot (the sole - e.g. the orange colour of the Garden Slug sole), but sometimes also of the mucus !
The first steps are the hardest ;-) ! Keep going !