Pop-up ad

Pop-up ads or pop-ups are forms of online advertising on the World Wide Web. A pop-up is a graphical user interface (GUI) display area, usually a small window, that suddenly appears ("pops up") in the foreground of the visual interface. The pop-up window containing an advertisement is usually generated by JavaScript that uses cross-site scripting (XSS), sometimes with a secondary payload that uses Adobe Flash. They can also be generated by other vulnerabilities/security holes in browser security.
A variation on the pop-up window, the pop-under advertisement, opens a new browser window under the active window. Pop-unders do not interrupt the user immediately, but appear when the user closes the covering window, making it more difficult to determine which website created them.
History.
Pop-up ads originated on the Tripod.com webpage hosting site in the late 1990s. JavaScript provided the capability for a web page to open another window. Ethan Zuckerman claims he used that capability to launch advertisements in separate windows as a response to complaints from advertisers about their ads appearing on pages with sexual content. Zuckerman later apologized for the unforeseen nuisance pop-up ads had evolved into.
Pop-up blocking.
Web development and design technologies allow an author to associate any item on a pop-up with any action, including with a cancel or innocent looking button. Because of bad experiences and apprehensive of possible damage that they may cause, some users do not click on or interact with any item inside a pop-up window whatsoever, and may leave the site that generated them or block all pop-ups.
Opera was the first major browser to incorporate tools to block pop-up ads; the Mozilla browser later improved on this by blocking only pop-ups generated as the page loads. In the early 2000s, all major web browsers except Internet Explorer let users block unwanted pop-ups almost completely. In 2004, Microsoft released Windows XP SP2, which added pop-up blocking to Internet Explorer. Most modern browsers provide pop-up blocking tools; third-party tools add other features, such as ad filtering.
Fake close buttons.
Users of websites and web applications continuously experience unwanted pop up ads through the course of their normal interaction with a web browser. Ordinarily, users respond by dismissing the pop-up through the "close" or "cancel" feature of the window hosting the pop-up. Because this is a typical response, some authors of pop-up advertising depend on this, and create on-screen buttons or controls that look similar to a "close" or "cancel" option. When the user chooses one of these "simulated cancel" options, the button performs an unexpected or unauthorized action (such as opening a "new" pop-up, or downloading an unwanted file on the user's system).
Circumventing pop-up blockers.
A "hover ad" or "in-page pop-up" uses JavaScript to combine a banner ad and a pop-up window that appears in front of the browser screen. JavaScript imposes an advertisement over a webpage in a transparent layer. This advertisement can appear in a variety of forms. For example, an advertisement can contain an Adobe Flash animation that links to the advertiser's site. An advertisement can also look like a normal window. Pop-up blockers cannot block the ad because it is a part of the webpage, but it can be blocked with third-party ad blockers such as AdBlock and Adblock Plus, or by using custom style sheets.
URL redirection.
URLs are sometimes redirected to advertisement pages by URL redirection.
URLs are sometimes opened in a new tab and then the content of the old background tab will be replaced with an advertisement page by URL redirection, other times it switches the tab the user is on to the advertisement tab. Adblock Plus, uBlock and NoScript cannot block these redirects.
Techniques.
Pop-under ads.
Pop-under ads are similar to pop-up ads, but the ad window appears hidden behind the main browser window rather than superimposed in front of it. As pop-up ads became more widespread and more intrusive, often taking up whole computer screen, many users would immediately close the pop-up ads that appeared over a site without looking at them. Pop-under ads do not immediately impede the view of content, but remain unnoticed until the user closes or minimizes the main browser window. 
Pop-under ad technology.
A pop-under ad involves two JavaScript functions introduced in 1995 with the Netscape 2.0b3 browser. Modern web publishers and advertisers use it to create a window in front of the users screen, load an advertisement, and then send it behind the screen.
// create a new window in front of the current site
window.open( URL, windowName[, windowFeatures] ); 
// push the loaded advertisement back behind the browser
window.focus();
Most modern browsers allow codice_1 to execute only if it was called by a user interaction (e.g., a mouse click) event handler. Any non-interactive calls (timer callback, load events, etc.) to codice_1 result in the new window being blocked.
To bypass this restriction, most pop-under ads trigger on a mouse click event listener attached directly to the document or the document's body. This enables catching all mouse click events that were not consumed by other click event handlers, and calling codice_1 without being blocked. For example, when the user selects a text, the mouse click triggers the mouse click handler attached to the document and a pop-under opens using the above code. Other techniques to bypass the codice_1 call restriction do so by "hijacking" mouse clicks.
Hover ad.
Hover ads, more commonly called in-page pop-ups, are a special type of pop-up ads created using Dynamic HTML, JavaScript and similar web browser technologies. Because they do not scroll with the web page, they appear to "hover" over the page, usually obscuring the content. Hover ads tend to be very hard to block by pop-up blocking software
because the hover ad window is an integral part of the HTML content of the web page. Thus a software filtering the content has no algorithmic means of recognizing and removing parts of
the content, either descriptive or procedural, that create, populate and manipulate the hover ad's window.
Patent controversy.
ExitExchange.com filed for a patent in 2000 on a subset of pop-under advertising called an exit pop. After years of controversy and numerous articles on the pop-under patent, the patent was awarded by the United States Patent and Trademark Office (USPTO) in April and June 2008. The respective patent numbers are and . Patent 7,386,555 is related to the method of opening an exit pop from a toolbar or software application on a computing device, whereas Patent 7,353,229 covers the method used to open an exit pop from an embedded script found within a media file (e.g., JavaScript code on a web page).
Copyright issues.
Copyright aspects of pop-up advertising are discussed in the Wikipedia articles Derivative works and Transformativeness. Both articles contain illustrations and links to examples of pop-up advertising.