A window is no longer just a frame holding a few panes of glass. Instead it consists of a custom-made glazing unit designed to a precise specification for size, style, performance and appearance. Frames are typically made of the rigid plastic called uPVC (unplasticized polyvinyl chloride), aluminium or wood (or a combination thereof), and glazing may consist of two (double) or three (triple) panes of glass enclosing a sealed space between them. These units can be adapted to all manner of designs and openings, in various colours and finishes.
Technological improvements continue to enhance the insulating and noise-proofing properties of modern windows, so what are the main features to look for?
Low-emissivity glass (low-E glass)
(Images courtesy of Green Spec)
This makes a significant difference to performance. The face of the inner pane is coated with a very fine layer of metal oxide, which permits the sun's short-wave radiation to enter the room, but reflects long-wave radiation back into the room.
Insulating spacer bars
The panes are separated around the perimeter by spacer bars. Traditionally these were made of aluminium, but this conducts small amounts of heat out of the room. Hence, performance is improved by using non-metallic spacers, such as glass fibre, rigid foam, or reinforced plastic, called 'warm edge' spacers.
Gas-filled units
Insulative efficiency can be boosted further by filling the gap between the panes with a low-conductivity inert gas, such as argon, krypton or xenon. This reduces conduction and convection of heat across the gap.
Double or triple glazing?
Other things being equal, the most efficient units consist of three panes of glass and a double gap. But such triple-glazed units are typically 20-40% more 
expensive than double-glazed units, and weigh up to 50% more. They are popular in severe climates, such as Scandinavia, and are demanded if construction is to the stringent Passivhaus standards. Low-E glass double units can achieve the same A ratings as normal triple units.
Ventilation
Replacing leaky old windows with modern units will make rooms more airtight, and can lead to increased condensation. Depending on the amount of existing ventilation, replacement windows may need trickle ventilators that allow air to enter and maintain a flow through the building. Usually an assessment of the entire house will show what system is best.
Frame design and sustainability
Plastic uPVC frames are very popular because they require little maintenance and are the cheapest option. But of course they are derived from oil-based raw materials, and are not appropriate for some buildings. It is now possible to recycle old uPVC frames, so ensure your installer is aware of this!
Wooden frames, particularly hardwood, can match the performance of metal or plastic, and are a more sustainable product, but are generally more expensive and require more maintenance. As cheaper alternative, wood/aluminium composites are now available, where the metal protects the external surface of the wooden frame.