In Dungeons and Dragons and derivative adventure games (notably including almost every Roguelike in existence), green slimes are puddles of greenish, parasitic goop. Mostly immoble, a slime is just barely able to ooze in the direction of sensed movement (this is usually explained as "they can feel vibrations through the floor/wall/air").
This is not to say that green slimes are harmless or simple to destroy. Normal attacks rarely damage a slime—how do you cut or bludgeon a puddle? Worse, there's a risk of splattering green slime on yourself or your fellow adventurers. This is bad.
Green slimes stick to almost any living thing and grow by converting whatever they're attached to into more slime. (You've all seen The Blob, right?) An unfortunate adventurer can be converted entirely into more slime in fairly short order; the usual time given is between a few minutes and an hour. The process of breaking Our Hero down into slurry is generally irreversible and unstoppable, short of a wish, divine intervention, or amuptation of the affected limb.
Most slimes are extremely vulnerable to fire.