For RDL/stiff legged deadlifts you should only have a slight bend in the knee and focus on moving your ass back during the eccentric instead of letting your knees get forward. It's a great exercise that will primarily load your hamstrings and glutes, which is why every* girl you see in the gym are doing them. Glutes and hamstring are part of a normal leg day.

For a deadlift you knees will have a greater bend and move slightly forward which puts your body in a better position for maximal force output. You can load it a lot heavier and you're putting more tension on your quads. Deadlifts isn't as much of a leg exercise as it is a "your entire posterior chain" exercise.

*not every, just a whole lot