Woods are so called because, traditionally, they had a club head that was made from hardwood, generally persimmon[1] but modern clubs have heads made from metal, for example titanium, or composite materials, such as carbon fiber. Some golf enthusiasts refer to these as "metals" or "metal woods" but this change in terminology is not strictly necessary, because while the material has changed, the style and intended use has not. The change to stronger materials has allowed the design of the modern woods to incorporate significantly larger heads than in the past. Because of the increase in club head size, in 2004 the USGA created a new stipulation for the size of the club head. The legal maximum (by the rules of golf) of a wood is 460 cubic centimeters[2].
Woods are numbered in ascending order with the driver, or 1-wood, which is designed to hit the ball the greatest distance, having the longest length and lowest loft, typically between 7 and 13 degrees. However there is no industry standard and some overlap occurs. With the typical loft of a 2-wood being 12-13o a 1-wood and 2-wood could have the same loft. Other differences in club design, such as head size, face roll, weight distribution, lie angle, or club length, could also differentiate a 12o 1-wood intended for tee shots and a 12o 2-wood intended for fairway shots. Most club manufacturers’ matched sets include 1-3-5 numbered wood. Even numbers are less common with modern clubs. The number of the club is mainly a reference for the player to easily identify the clubs. Other identifiers have been utilized such as strong and plus to differentiate various lofts within a line of clubs.
Woods generally fall into two classes, drivers and fairway woods, with a traditional set of 14 clubs comprising a driver, two fairway woods (3 and 5), ten irons and a putter[2]. Many modern sets tend to include hybrid clubs, which combine some of the characteristics of a wood and an iron, to replace the 5 wood and longer (3 and 4) irons.
With metal (usually steel), the designer can easily make the clubhead larger or smaller, add runners to the sole, or build out the toe--all in an effort to make the club perform better. In other words, to help you hit the ball straighter and with more confidence. Steel also has a higher strength-to-weight ratio than wood, allowing the club maker to use less material without sacrificing strength. The result is a lighter, easier-to-swing clubhead. Also, to help the club perform better and be more forgiving, weight is moved to the perimeter of the clubhead to help it square at impact and be more stable on off-center hits.
adapted from learnaboutgolf.com and wikipedia
