The two most common types of wood used on a banjo neck are mahogany and maple. There are also banjos made of walnut, rosewood, white oak, cherry, and more - but mahogany and maple are by far the most popular. The neck wood used on a banjo is one of the most important tonal decisions made.
Understand what the tonal differences are between a banjo made with a maple neck and one that is made with a mahogany neck.
Maple is a very hard and dense wood. This wood when used on the neck of your banjo will give the instrument a very bright tone. This is generally the preferred neck wood/tone for traditional bluegrass banjoists. This is most likely due to the fact that Earl Scruggs, the king of bluegrass banjo, played a maple banjo and had a very bright tone.
Mahogany is a much less dense wood than maple. When used as the neck wood on your banjo it will give the instrument a much warmer tone than maple. Some feel it is a sweeter tone. Bela Fleck plays a mahogany banjo and gets a very warm tone from his banjo. J.D. Crowe also played a mahogany banjo and was able to get a more bluesy tone.