For women, "Ralph Lauren Collection" is the top, high-end. One or two notches down are the "Ralph Lauren Black Label" and "Ralph Lauren Blue Label" lines, which are sort of diffusion lines but not really. The difference is that the Black Label price points are higher than the Blue Label.
Then, as softgrey mentioned, lower still is "Lauren By Ralph Lauren" which is the better priced line for bigger department store distribution.
For the men, "Ralph Lauren Purple Label" is the flagship line. Then comes "Polo Ralph Lauren" which is less pricey.
"Polo Jeans Co." is for both men and women and the cheapest as well.