Start with the skill level, if you are a scratch golfer to a 5 handicap, than the type of irons you would be looking for is a blade style that is made out of forged steel.
Forged golf clubs offer a softer metal than a stainless steel, and it allows you to have more feel for the ball. Blade style irons also allow you to shape your shots with greater ease than a more forgiving golf club. Some great examples of this type of club are the Titleist 695 irons, or the Callaway X Tour irons. The Cost of these ranges from $800-$900 depending on options.

6-12 handicap players should be looking for a club that offers some forgiveness and also the ability to work the ball, but still have some margin for error. Some great examples of these irons are the NIKE CCI irons and the Callaway X-20 Irons. These clubs range from $650-$750

12-18 handicap players should be looking into a game improvement style golf club.
A game improvement club is designed with large perimeter weighting, and larger sole.
The purpose of perimeter weighting is to push the weight of the club around the club face, to allow the center of the face to be as forgiving as possible i.e. Large Sweet Spot.
Clubs that would fit that description are the Cobra FP Irons, Callaway Big Bertha Irons & Tour Edge QL Irons. Price range $300-$550

18+ handicap players should be looking for an ultra game improvement club.
These clubs vary in terms of materials and price. The concept of these irons sets is to make golf easy and enjoyable for the average golfer. The manufacturers do this in many ways. First of, almost all of the clubs in this category have a very wide sole, which makes it easier to get the ball airborne? Other features this category offers is the size of the head. The head size is much larger than a blade style to create a larger sweet spot. A good theory is that the larger the head the larger the sweet spot. This category also offers other materials such as Titanium and Polymers to increase strength without increasing the overall weight of the club. Great examples of these clubs are the Callaway Fusion Wide Sole, Cobra S9 & Nike Ignite Irons. They range from $400-$1,000 per set.

Other points to consider are golf shaft options.
You have 2 options to choose from Steel or Graphite. Steel shafts are less expensive than graphite. They also offer more control for you shots, and give more ability to work the ball than a graphite shafts. Steel shafts are also known to be a stiffer shaft and offer a strong swinger a better platform to perform.
Graphite shafts are more expensive than steel, but are much lighter. The difference in weight is almost half. A light weight shaft is around 55 grams opposed to 120 grams in Steel. The benefit of graphite shafts are for those golfers who have smooth tempo swings.
You would let the club do the work for you when using a graphite shafted club. Graphite shafts are very popular with ladies and seniors due to there slower swing speeds.

Last but not least, hit the golf clubs to be sure which is best for your game.
Manufactures have Demo Days at most all local golf course or golf shows so the end consumer can try the latest golf equipment products available.