Cone Yarn?

While shopping through cone yarn on ebay I see a lot of them with numbers such as 2/24 in their description. Can someone please explain to me how to decipher them. I know it has to do with the ply and/or thickness. Which is which, and is there a site that clearly explains this? I can’tfins one.

IN FACT: This type of information would be good to have here among the teaching info on this site. Hint/hint.

It would save me a lot of time looking through the cones if I knew how to read the numbers because 9 times out of 10 the description does not tell you if the yarn is fingering, lace, DK, etc. and it is hard to tell from the picture.

ALSO: Does YPP stand for yards per pound?

YPP does stand for Yards Per Pound.
Generally, light fingering and lace is sold like this. One number represents the ply, the other the thickness or weight as in 8/2, which is size 8 with 2 plys. The larger the number for the thread size, the smaller it is… so 5/2 and 3/2 are thicker, 8/2, #10, 50/2 is super fine.
Hope this helps… Oh yeah, it makes sense that the smaller the thread, the more YPP there are.