Base coat: To ensure adhesion between the natural nail plate and the colored nail polish. Use in one, thin layer under the color. Features: usually clear, dry quickly to a dull, stickey surface.
Top coat: Use on the top of the hal dried, colored nail polish to ensure protection and high shine. Great to fit flat nail designs, like stones, glitter... Features: Usually clear, slower dry (there are speed-dry top coats in the market), dry to a shiny, smooth surface.
Does 2in1 polishes work? I dont think so. As the two opposite functions of theese opposite properties dont work at the same time. I believe 2in1 (base and top coats) are not for professional use.