The new applications are usually added by use of Sun's Java programming language. This language allows a programmer to write a utility -- perhaps an interesting animation, a spreadsheet or a game -- and have it run on any computer with the right software installed (for instance, later versions of the Netscape Navigator browser, or Sun's own HotJava browser). Compare this with the usual situation where a different version of software is needed depending on what hardware and operating system you have chosen!
Video clips and sounds often require some extra software to be installed before they can be viewed or heard, although the trend is towards Web browsers with this software included. Though undoubtedly more interesting than simple pictures and text, and for some purposes very useful (such as for tourist sites; see The Virtual Tourist elsewhere in this issue), these additions are generally quite large and so take some time to download, especially for potential customers on a dial-up link. Java 'applets' (programs to be included within a Web page) are often smaller, and so do not have quite such an impact on the perceived speed of a site.
Although video and sound clips do not suffer from the same security stigma as Java, at the moment it is often true that sites have added these frills without forethought or consideration for the impact on users with slow links or without the latest version of browsers. It is quite possible to make a page that will still be viewable by someone whose browser does not, for example, support any form of animation. Such viewers need to be considered by any designer who is interested in reaching as many people as possible with their sites.