Not all pop-ups open when the Web page loads, though. Some activate when you click on a link or when you scroll your mouse over an activation site on the Web page. A good pop-up blocker can detect the code for these ads as well, though some are only able to deactivate a pop-up after it starts to open. If you've ever seen a pop-up quickly appear and disappear when you have a pop-up blocker activated, that's what happened. The blocker detected the ad after it activated, then disabled it.
But not all pop-up blockers are created equal. Some of them are very good at blocking nuisance pop-ups while leaving helpful pop-ups alone -- they're Intelligent pop-up blockers. In general, most intelligent pop-up blockers only disable pop-ups that open when a Web page first loads into your browser -- any pop-up that opens as a result of the user clicking on a link remains unhampered.
Trainable pop-up blockers rely on user input to determine which pop-ups to deactivate and which to leave alone. No two pop-up blocking programs are exactly alike, but in general the blocker asks the user if he wants to disable a particular pop-up when it first appears on the user's screen. If the user clicks on "Yes," then the blocker puts the offending pop-up on a black list, or a list of ads that should be deactivated in the future.
If you choose "No," the blocker puts the pop-up on a white list, which means it won't attempt to block the pop-up window again. While this kind of blocker is very easy to customize, it can become frustrating for the user as he makes choices for each and every pop-up. Also, if the advertiser alters the original pop-up in some way, it may slip past the blocker.
Serial pop-up blockers block every pop-up ad they're capable of uncovering. These blockers seek out every variation of the code used to generate pop-up ads and deactivate it whether the pop-up contains advertising or not. While these blockers usually ensure the user a pop-up-free experience, they can also cause complications when navigating through certain Web pages.
In the next section, we'll look at why you would want to disable a pop-up blocker.