Double glazed windows help in controlling the inside temperature keeping it warmer in winter and cooler in summer, making it a smart way to reduce heating and cooling costs. Double glazing is quite effective as it stops all heat transfer from one side to another. It also stops UV damage.
They are great for hot and cold environments. Depending on whether you are keeping hot or cold out the insulated glass units are built differently. That's the basics it actually can be quite complicated once you throw all the options on the table.