The difference between a lady shaver and an epilator is that the lady shaver is used to just trim hair from the skins surface and shave the hair as normal.
But the epilator is designed to pluck hair from the skin and not just shave it.
So you shave and remove part of the hair from your skin with the lady shaver while the epilator actually removes the hair from your skin completely.
New hair can grow back but the hair is actually removed by plucking it with the epilator so that you have smoother skin.
If you want smoother skin you would use an epilator and if you just want to shave the hair from your skin such as your legs then you would use a lady shaver.