A conventional deadlift targets the hamstrings, glutes, and quads. The main difference is that the stiff-legged deadlift require much less, if any, involvement of the quadriceps. It mainly works the hamstrings, glutes, and erector spinae.
A deadlift is a hip dominat exericse that utilizes the glutes and hip flexors. A stif leg deadlift (your legs are not really stiff), is also a hip dominant exericse focusing more on hamstring development.
stiff legged deadlift..I try to feel it more in the lower back if I can. I used it as the lower back exercise part in my routine. I do not know many lower back exercises