Slugs are the shell-less cousins of snails. They are not picky eaters, and will eat many kinds of plants including most crops. There are many species, but the grey garden slug is often the most damaging.
Slugs will eat almost any part of a plant, including seeds, roots, stems, and leaves. They are most damaging early in the season when the seed or seedling is most vulnerable and when cool, wet conditions prevail.