As I was browsing through coins online I noticed some abbreviations that I was wondering about. I was looking at HSN.com at proofs and things and I was wondering what these mean and what the difference is between them? I know that they are the grade of the coin, (i.e. PR 70) but what makes PR different from MS, SP, or RP?

MS
PR
SP
RP

I recently ordered the 2007 and 2008 PR70 Pres. Dollars from there and I just wasnt sure the differents between the grades. I know that the higher the number, the higher the grade. Can somebody help me out?

MS= Mint State
PR= Proof
SP= Satin Proof
RP= Reverse Proof

MS coins are made for general circulation
Proof coins are made with special dies

To the naked eye, there is no difference between a 69 or 70 in my opinion