Well, a leave-in conditioner usually moisturizes, so they're both the same thing. Unless the conditioner being used somehow dries out hair, which in weird cases does happen, I think, or leaves hair feelings crunchy.
And yes! You can use a conditioner as a leave-in. I do this sometimes with my Suave Rosemary Mint conditioner. I only use a very tiny bit, though. Too much and I might make my hair too oily. If I use the right amount, it leaves my hair very soft and slick.