Advertising on web sites is fair enough; media providers want some return on their effort. Sometimes the ads are amusing and they add sparkle to pages. But it becomes seriously, seriously, annoying when a video clip starts playing when a page is loaded. Adblock and Flashblock to the rescue!
Of the two extensions, Flashblock is the simpler to use. It just replaces an embedded Flash movie with a still image. You can still click on that image to play the movie. Adblock doesn't do anything when it's installed. You have to mark individual elements to block or specify a regular expression filter to identify these elements. In theory, Adblock can block all advertisements but have to configure or download your filters while Flashblock doesn't block all Flash movies (sometimes the references are tricked up), so it makes sense to combine the two extensions for control and convenience.