A popup is a new browser window which is opened by a website. The most classic use of the popup is in Internet advertising, where popups are used to attract the attention of potential customers. Some websites also utilize popups as part of their support and help sections, allowing users to click on a “help” button for assistance which pops up, rather than taking the user away from the page that he or she is navigating. Many web users regard popup advertising as a major bane of their existence, as these ads can be extremely irritating.
It's hard to determine whether advertising or more legitimate uses of popup ads came first. In the case of advertising, popups are usually bright and garish, sometimes with flashing colors, and a site may open a cascade of popup ads when a user accesses it. Many companies also design popups with misleading buttons, causing users to click the wrong button when they try to close the ad, which can trigger the appearance of more popups, or start a script which performs an operation on the user's computer.
Certain types of websites are notorious for creating a deluge of popups for unwary users. Sites which provide downloads of material of dubious legality often have popups, and popups are also very common on pornographic web sites. In some cases, these popups also include scripts which can harm a user's computer, and these sites should be navigated carefully, if at all.
In the sense of website assistance and help, popups can be very useful. For example, when someone fills out a complex form, the form may have buttons which can be pushed for assistance. When these buttons are pushed, a popup appears to provide help, so that the user can stay on the page rather than navigating back and forth. Popups can also contain tools like calculators which can be used to fill out the form.
A basic browser usually comes equipped with an option to block popups which will automatically stop popups, sometimes with a prompt at the top of the page to alert the user. This prompt provides an option to allow popups from specific web sites, so that users can permit help windows and other types of useful popups to open.