This question can have multiple answers depending on your location. Advances in glass has also meant there are many options for thermal insulation these days.
Double glazed glass provides good thermal insulation. Due to the air space between the two panels, less heat is transferred between the exterior and interior of your home. This means that in winter less heat is lost to the outside, and in summer less heat is allowed inside.
There are other glazing options available which can help with thermal insulation. Please look at the energy efficiency section of the website for more details.
Glass however is not the only consideration when it comes to thermal insulation. The type of material will also impact on thermal insulation, timber will generally have better insulation properties than a commercial aluminium frame for instance. Please speak to your local Wideline showroom for further information on thermal insulation solutions.