Good topic to discuss! You're opening up a pandora's box when you ask questions like you did.
The variations and styles are almost infinite and confusing to say the least.
In poking around google, I ran across a great article on steel blade types, the advantages of each, and their disadvantages:
http://bestpocketknifetoday.com/discove ... ife-steel/" onclick="window.open(this.href);return false;
The numbers are confusing yes, but IMO, studying those steel types is the best way to make an informed decision. One such steel, S30V seems to offer the best overall performance(again IMO):
"CPM S30V (often simply referred to as S30V) steel has excellent edge retention and resists rust effortlessly. It was designed in the US and is typically used for the high-end premium pocket knives and expensive kitchen cutlery. Dollar for dollar, this is generally regarded as one of the finest knife blade steels with the optimal balance of edge retention, hardness and toughness. Pretty common these days and one of our favorites."
Kershaw makes knives with that kind of steel: Kershaw Knives: Camber - Stonewashed Finish - Black Aluminum - 1678. Sells for $99.95 on one website.
NOTE: I'm NOT recommending that knife. Never had one. Just made that as example of a quality knife in the general price range.
If I were in the market for a quality, folding defensive knife, I'd study that link, and decide on the 2 or 3 quality steel types I'd like in a knife, then start searching the knife websites. I'm sure your choices would be endless, and I don't envy you in your search, but good luck!
Hope that helps.