Irons are so-called because their clubheads are made of metal. Of course, "woods" are now also made of metal, but that's a relatively recent development. Irons have featured metal clubheads (steel, these days) for centuries.
The clubheads of irons are thin from front to back, and the clubfaces are grooved to impart spin on the golf ball. Accomplished players might choose a "muscleback" or "blade" style of iron, whereas beginners and most recreational players will want a "cavity back" style.
The difference is that a blade-style features a full back on the rear of the clubhead, whereas a cavity back is exactly that: the rear of the clubhead is, to a certain degree, hollowed out. This creates an effect known as "perimeter weighting," which is helpful to less-accomplished players. Beginners should always choose irons described as "game improvement" or "super game improvement," as these provide the golfer the most "help."
Irons: Set Composition
A typical, off-the-shelf set of irons will include a 3-iron through pitching wedge (advertised as "3-PW"), 8 clubs total. The clubs are identified by a number (3, 4, 5, etc.) on the sole of each club, except for the pitching wedge which will have a "PW" or "P." Other irons may be available for purchase separately, including a 2-iron and additional wedges (gap wedge, sand wedge, lob wedge). None of the additional clubs are necessary for beginners, and especially not the 2-iron. 1-irons used to be available, too, but are now virtually extinct.
Relative newcomers to golf shops are sets called "blended sets," or "hybrid iron sets." These sets replace the traditional long irons with hybrid clubs, and fill out the set with cavityback mid- and short irons. (See more about golf sets and which clubs to carry)
Iron Loft, Length and Distance
As you go through the set, from the 3-iron to the pitching wedge, each iron has a little more loft than the previous, and a little shorter shaft length than the previous, so each club (going from 3-iron to PW) hits the golf ball little less distance than the previous. That is, a 5-iron has more loft, a shorter shaft, and produces shorter shots than the 4-iron; the 4-iron has more loft, a shorter shaft, and produces shorter shots than the 3-iron. The pitching wedge has the most loft, the shortest shaft, and the shortest distance in the traditional 3-PW iron set.
The yardage gap between irons is generally 10-15 yards. Your 3-iron, in other words, should produce shots that are 10-15 yards longer than your 4-iron. The specifics of this gap depend on the player, but the gap should be consistent from club to club.
Also, as you move through the set to the shorter, more lofted clubs, the resulting shots will have a steeper trajectory; shots will rise at a steeper angle and fall at a steeper angle. That also means that a ball hit with the 8-iron, for example, will roll less once it hits the ground compared to a ball hit with a 4-iron.
Long, Mid- and Short Irons
Irons are generally categorized as long irons, mid-irons and short irons. Long irons are the 2-, 3- and 4-irons; mid-irons, the 5-, 6- and 7-irons; short irons, the 8- and 9-irons and pitching wedge.
For most amateurs, the short irons are easier to hit than the mid-irons, which are easier to hit than the long irons. Without getting too technical, the reason is that as loft increases and shaft length decreases, a club becomes easier to master. A shorter shaft makes a club easier to control in the swing (think of baseball where a batter will "choke up" on the bat - essentially, shorten the bat - when he's simply trying to make contact rather than swing for the fences). More loft helps get the ball airborne and adds a little more control to the shot.
Distances with Irons
Learning your distances - how far you hit each club - is much more important than trying to hit each club to some predetermined "correct" yardage. There is no "right" distance for each club, there is only your distance. That said, a typical male recreational golfer might hit a 4-, 5- or 6- iron from 150 yards, while a typical female might use a 3-wood, 5-wood or 3-iron from that distance. Beginners often overestimate how far they are "supposed" to hit each club because they watch the professionals blasting 220-yard 6-irons. No matter what the commercial says, you are not Tiger Woods! Pro players are in a different universe; do not compare yourself to them. (See "how far should I hit my golf clubs?" for more on this.)
Irons can be played from the teeing ground, using a golf tee, and it is often appropriate to do so. On a par-3 hole, for example, you will probably use an iron on your tee shot. Or you might use an iron off any (or even every) tee in order to have better control over the shot.
But most of your iron shots will come from the fairway. Irons are designed with divots in mind. That's why they have a leading edge that is somewhat sharply rounded. If you take a shot with an iron and dig up a chunk of turf, don't feel bad. Maybe you dug up too much turf (which is called a fat shot), but it is entirely appropriate to take a divot with an iron played from the fairway.
That is because iron shots are played with the ball positioned so that it is struck on the downswing. That is, the club is still descending when it makes contact with the ball. See the article "Hit Down, Dammit!" for more about the concept of irons being designed to strike the ball with a descending blow. And see the article "Setup for Success" for imformation about proper ball placement in your stance.
Knowing which iron to use in which situation is mostly a function of learning how far you hit each club. But trajectory also often comes into play. If you need to hit the ball high - to get over a tree, for example, or to make the ball land "soft" on the green (meaning hit the ground without much roll) - you would choose one of the higher-lofted clubs. So learning the trajectory of each of your irons - how high the ball climbs, and how quickly it climbs, with each iron - is another important factor.