While recent studies indicate most people find pop-up ads annoying, only about 20 percent of online surfers were using ad or pop-up blockers last year.

Pop-ups are created by a script inside a web page. When you visit a site containing one of these scripts, it instructs the browser to create a window for the pop-up. There are several different types of pop-up ads including:

• Mass Attack Pop-ups - Advertisers try to overwhelm pop-up blocking software by opening as many pop-up windows as quickly as possible.
• VBScript Pop-ups - Users can't do anything until they interact with the pop-up, either by clicking on it or closing it.
• Timed Pop-ups - Using a delayed timer, they try to trick blockers into thinking there is no pop-up activity.
• Mouse-Over Pop-ups - Created when users roll over an ad or scroll down a page.
• Full Screen Pop-ups - Take up the entire screen, either with or without the toolbar visible.
• Industry Standard Pop-ups - Used by most ad agencies. When visitors open a web site, a separate ad pops up.
• Pop-under Ads - Less intrusive because the window opens underneath the active window.
• Floating or Overlay Ads - Not considered an actual pop-up ad. These ads remain on the page until they disappear or a visitor leaves the page. They usually cannot be closed or minimized.

While some sites use pop-ups for less than legitimate reasons, many reputable sites like The New York Times, Weather.com and CNN.com use pop-ups to generate revenue.

Even though most people find pop-ups and floaters annoying, they are difficult to ignore. And while they cost more than standard banner ads, they also deliver higher click-through rates.

Some web browsers have built-in functions for blocking pop-ups and standard banner ads. Mozilla and Opera are two such free browsers. Internet Explorer also offers ad blocking technology, but only if you're using Windows XP and have Service Pack 2 installed. Software makers such as Symantec have pop-up and ad blocking features in many of their products as well. And there are many free pop-up and ad blockers available for download like Pop-Up Stopper, STOPzilla and Pop-Up Defender.

Most of these browsers and software also allow users to customize settings - allowing pop-ups or banner ads from certain sites.

However, developers are finding ways around the most popular ad blocking and pop-up filters. Technology is now being developed to detect whether or not a computer is using a pop-up blocker. If one is detected, a floating ad is delivered to the page instead.

If you're an advertiser, deciding whether or not to use banner ads or pop-ups can be a difficult choice. Some potential customers may not see your message due to ad blocking software. However, most sites, including Locator Online, offer promotional opportunities to supplement banner ads like part or company listings, links to articles and photos, and Parts Wanted e-mails. Other sites might offer sponsorships or paid positions on listings that are not blocked by software. The key is to create a well-rounded marketing plan to reach all customers including those using ad or pop-up blocking software.