Blanket question

What is the difference between a rain sheet and a waterproof sheet? Why does it seem like I can't find any rain sheets? Does anyone have a particular brand that is tried and true??

I think a waterproof sheet isn't as waterproof as a rain sheet.. if that makes sense. I also think that a rainsheet would last longer and stay more rain- proof a lot longer.
Here's what I got on dover for searching 'rain sheet'

Dover Saddlery - Search Results for rain sheet[[SLI_IDC]]&ids=1018838641

A rain sheet would be sort of like a rain coat for a person...I've heard from alot of people that they don't like them 'as much' as a good waterproof sheet. The ripstop material in alot of waterproof sheets are a winner. Rain sheets are really expensive as well. I know for myself, I prefer the waterproof the rain sheets; they seem to hold up way better with regular turnout. Again, that's just my experience (and friends), so depending on the brand, and what not, your results might be different.

Does anyone have any experience with Turtleneck brand horse sheets?