The pop-ups that come on your screen as a web page loads are usually caused by something called JavaScript. This is a simple programming language that can be integrated into the HTML used to build web sites.

Some pop-ups are useful. Sometimes a user clicks a link on the Web page and a small window will open asking a user to fill in information or giving a user more information. Music sites often use pop-ups to play tunes in a specialized player window. This can be a completely legitimate use of pop-ups and is quite different from the pop-up advertisements that appear unbidden.

JavaScript can control the size of the window and the elements displayed. The status bar or the menus can be hidden, making it impossible to navigate away from the pop-up. Your only options are to close the window using the 'x' box in the top right corner or to click on the ad and be taken to the advertiser's web site. Annoying, huh?

Some pop-up windows are either poorly programmed or intentionally malicious. The JavaScript will create multiple windows, and when one of those windows is closed more windows will open. The only way out of this endless cycle is to close the browser.

A recent type of pop-up is created using software called Flash and can even be made to follow your mouse around the screen as you move it.

In addition to pop-ups created with JavaScript and Flash, 'adware' can create pop-up ads according to the content on a web page. Adware is usually just advertising-supported software which displays ads when you use that program. Sometimes, however, 'adware' refers to small hidden programs that deliver targeted advertising to your web browser. These programs may look for certain keywords on a web page and generate advertisements based on those words.

Another type of pop-up is the pop under. Rather than appearing on top of web pages they're hidden until you close the browser window. These are a little bit less intrusive than pop-ups but still an unwanted nuisance. There's no end to the cleverness of advertisers!

The main problem with pop-ups is the nuisance they cause, and almost every web browser has some kind of pop-up killer included these days.

There are also many specialized pop-up killer utilities that run in the background and require no user intervention, most available free of charge. So, download one today. Unless you just enjoy advertising.