Sorry: didn't think about the fact that "double glazing" might not translate well!

It's also known as insulated glazing or double-pane: basically, windows with two (or more) sheets of glass, separated by a gap of air (or, in more-fancy models, argon gas). This provides significantly more insulation than a single pane of glass alone, and so dramatically improves the energy-efficiency of the house (heat loss through windows is a big way of losing heat from your house; after loft insulation and draft exclusion, it's one of the biggest ways to have an impact).

Here in the UK, a huge number of houses were built during the rapid expansion of the cities in the 50s and early 60s, driven by the reconstruction of war-damaged areas and our (more-limited than the US) "baby boom". Later, in the late 80s, 90s and early 2000s, as energy prices steadily climbed (and coupled with increasing concern for energy waste and environmental impact), there became a significant market for retro-fitting the windows of these houses with new, modern, double-glazed models. Into this market gap sprung a large number of companies selling double-glazing installations. In order to spread the message about the benefits of their product, and to compete with their many rivals, double-glazing firms employed shady marketing tactics, which quickly became among the most-hated. Having a door-to-door double-glazing salesman appear at your door was just slightly worse for most people than, say, having somebody at your door trying to tell you about the superiority of their religion.

As well as door-to-door sales, double glazing companies famously used cold calling, dodgy product comparisons (I was instructed, for example, to pay special attention to the houses of people who'd had aluminium-framed windows installed: these were originally sold as more-durable than the PVC-framed alternatives, but I was to spread rumours that their ability to stop condensation and leaks was less-good than PVC-based windows), and aggressively-marketed fake discounts ("I shouldn't be doing this, but if you sign here today, I can put it through on a special offer and get you 25% off," says the salesman, knowing that he already put the price up 40% in anticipation of using this tactic).

By the time I was (in desperation) doing this job, in about 2001, most houses already had double-glazing - either because they'd had it installed in the 90s or because they were new buildings (and UK building codes require energy-efficiency standards to be met in new construction). In the early to mid 00s, many double-glazing companies collapsed or merged, as the market dried up and they were only able to sell to the smaller market of construction companies. There are few single-glazed homes left in the UK, and most of them are so because their owners simply don't want or can't afford double-glazing (it's true that it saves you money in the long run, but the outlay of replacing all of your windows makes it among the least cost-efficient ways to save money in the short term). This made it a hard and hostile market, and for somebody like me - too honest to be a salesman! - it was impossible to become good at.

And so, I quit. And every time I mention this job that I did, once, people look at me with the same kind of revulsion that they usually reserve for traffic wardens and politicians.

tl;dr: Two panes of glass with an air gap between, for better energy-efficiency. Also; a history of double-glazing in the UK.