Pop-up ads are advertisements that show up in a new browser window. There's no one standard size for pop-up ads. Popup ads also vary widely in the amount of browser commands that show in the window.
Pop-up ads are just one form of internet advertising used by advertisers to get web user's attention.
Among these other types of ads are ads located within a page you're reading, ads that appear when you're clicking from one page to another (these are called interstitials), ads that appear when you've searched for a keyword or term, ads that are highly animated or appear as interactive games, ads that appear like stand alone websites, ads that appear in email, branded sites or sponsored webpages (the advertiser's brand appears on all pages and the page composition might have been modified to fit the advertiser's logo or brand colors/layout), paid listing, and many others.
The only limit really is ever changing online media technology and that's not much of a limit at all. Popups are just one part of this expanding universe of methods in reaching online consumers.
Are pop-up ads different?
A pop-up is an advertisement displayed in a new browser window. Pop-up windows come in many different shapes and sizes, typically in a scaled-down browser window with only the Close, Minimize and Maximize commands.
Pop-ups are simply one of many formats, alongside fixed spaces within a page, interstitials (between pages), search, rich media, microsites, email, sponsorships, listings and others. Pop-ups are simply part of this digital media mix. The reason they are used is that they work - that's why marketers continue asking for them.