Wet fly is an umbrella term for all fly patterns that are fished beneath the surface. With these flies you can fish in both rivers and stillwater for brown & rainbow trout, grayling, whitefish and char. This category includes smaller wet flies that usually imitates insects like mayfly and stonefly nymphs or caddis larvae. Bead headed flies have more weight and they sink faster which is an advantage when fishing faster current or deeper spots.
Sometimes successful wet fly fishing requires adding weight to your leader and indicator makes detecting strikes easier.