There are 2 types of springs. Progressive and Linear. Just for reference, the stock springs are progressive. Anyway, progressive springs are rate with the numbers you mentioned. For example.. 450-625. That means that as the spring compresses it gets harder to compress. So when it is at normal height it would take 450lbs to compress the spring 1 inch. As it is compressed more it will get harder to compress. So then it will take 625lbs to compress the same spring 1 inch. Follow?
Progressive springs are better for street driving because they offer a softer ride but respond well when you hit the corners. Linear springs are rated with a single number. Perhaps 650. Meaning it would take 650lbs to compress the spring 1 inch throughout it's entire travel range. Linear springs are better for racing applications because they are a good bit rougher but offer much better handling, cutting down on body roll significantly. Hope this helps.
Bob