From DIYinfo.org
Timber
As with all exterior timbers used in house building, there are certain rules governing the quality of timber used in windows. According to the approved standards, timber for windows must be rated Durability Class 1 or 2.
Hardwoods are generally not easy to work, besides which, shrinkage is not easy to predict. For these reasons, window manufacturers now prefer to work with some of the more stable timbers, such as western red cedar.
While it is not uncommon for made on site windows to be fabricated from Doualas fir (Oreaon), this species does not conform to the durability specifications, and so should not be used for windows.
Steel
Steel windows are much less common these days than they once were, but as suitable timbers become both more expensive and scarce, the use of steel windows in houses probably will increase.
The metal of the newer steel windows is protected by hot dipped galvanising, then covered with a primer, undercoat and gloss paint. Older steel windows are not galvanised and many rust.
Steel windows are strong and have a slim appearance. They are available in a wide range of types, but because the metal is a poor insulator, it attracts airborne moisture which we see as condensation. Regular maintenance is necessary to prevent old steel windows from rusting and timber subframes from rotting.
Aluminium
Most aluminium windows are made to measure in a wide range of types. They have a slim appearance and need little maintenance apart from washing down. Usually, they are set in timber sub-frames which need regular treatment with paint, preservative or varnish.
Like steel windows, they can suffer from condensation, although there is much less risk of serious damage to the metal itself.
Plastic
Although many windows made from other materials incorporate some plastic parts, this material (unplasticised poly vinyl chloride -UPVC) is not yet widely used for making complete windows. Those that are made, will not corrode, warp or rot and need only regular washing to keep them maintained. A major drawback is their lack of resistance to intruders.