They come in the night to do their dirty work — chewing away on plant parts, leaving huge ragged holes on leaves and feeding underground. These mostly nocturnal shell-less snails can be devastating to a garden. They've never met a lettuce or cabbage leaf they didn't like, and they particularly enjoy hostas, which happen to be located (conveniently for the slugs) in the shady cool environment they need. The succulent growth of spring and early summer is their favorite menu.
Many of the usual slug-control methods have their downsides. Most slug baits are toxic to pets and fish (as well as humans) so they have to be used very carefully; products containing iron sulfate are safer. Sharp surfaces — such as with diatomaceous earth — cut slugs' soft bodies, resulting in deydration and death, but only when the material isn't wet. Pans of beer attract them — and very efficiently; you have to replenish the pan every day, and if there are roaming neighborhood dogs, fashion a sort of lid or barrier to keep them away.
During the day slugs hide out in moist, cool, dark areas such as beneath plant litter and flower pots. Holding slugs at bay is a matter of reducing the number of daytime hiding places they might find and doing a bit of regular housekeeping. It's easier to control them in a sunny garden; in a shade garden where some of their favorite foods, like hosta, are located, you'll have to use more than one method.