A new category in golf, rescue (or hybrid) clubs are designed to replace harder to hit long irons. A typical set of irons would be 3-PW (8 irons) and many golfers find the 3, 4 and even 5 irons quite difficult to hit consistently well. This is where rescue clubs come in. They are a cross between an iron and a wood and they have dual characteristics. Firstly, they have the easy to hit properties of a wood with larger, more forgiving heads. Secondly, they have the same accuracy properties of iron with shorter shafts and iron-like faces for control. An added benefit of rescue clubs is that they are much easier to hit from poor lies or from the rough.
Most golfers should consider putting a rescue club or two in their bag. If you don't use your long irons that much then you can replace the 3, 4 and 5, but it may be worth starting with the 4 iron replacement. Better players can replace the 3 or even introduce a 2 iron replacement for more distance.
Rescue clubs can also be used off the tee and are very useful for longer par 3 holes, so do consider the higher lofted replacements if you prefer a wood to an iron.
The shaft of a rescue club is made from either graphite or steel. Graphite can provide the benefit of giving more club head speed as it is lighter and has more flexible properties. Steel can be more accurate. As a general rule most golfers from beginners to intermediate will benefit from graphite. Many tour pros use graphite, yet some use steel, so more experienced golfers should investigate the latter. Steel shafts are also cheaper to buy than their graphite counterparts.