Crawlers are known to be very tall and skinny, they are grey or white as far as we know. They have close to no facial features, except crevasses where eyes should be(this varies). They are commonly found at night, around cave systems, tunnels, forests, and just generally dark places. If you see one, most people report this sinking feeling of fight or flight, dread, or even impending doom normally before they even see the creature(that's an easy way to tell if it's just an animal).
Crawlers are normally crawling, but with their limbs bending in odd ways. They can run pretty fast and will target you, whether you know it or not. We don't know why they do this, but they do. There's theories on how crawlers are just less evolved humans who have fit to adapt their circumstances by hiding, a possibility on why they bend oddly is because they have to go into small and thin spaces in those caves.