Sunless tanners are products that give your skin a bit of color without the need for sun expoure. They have come such a long way since the original smelly, streaky orange formulations. While the active ingredient in sunless tanners is still dihydroxy acetone (DHA), newer formulations have added bronzers, fragrance, and other ingredients to help them glide more smoothly over the skin so there is little to no streaking. DHA interacts with a specific amino acid in the outer layers of your skin and that reaction creates the color called a "tan." It does not go beyond the outer layers of your skin. Interestingly, the same ingredient, is sometimes used to help create that golden brown look to bread that we eat- yum.
It is important to note that the “tan” you get from a sunless tanner is not protective against UV radiation so you still need to apply sunscreen and make sure to re-apply every two hours or more often as necessary, and follow the other concepts of “sun smart” behavior. There are lots of brands that work really well at all different price points. I like Coppertone, Clarins and Dauphin, among so many others.
Its always best to exfolliate first, and apply to knees, elbows and ankles last, just feathering it in these areas since the skin is thicker here and too much absorption of the product will leave them looking darker than you want.