Effects such as animations can make your pages more eye-catching, but generally at the expense of longer page download times. If you make your audience wait too long for a page to load, they may well leave your site. Once they have waited, it's important that they consider the waiting was worth it! If there is a good reason for an annimation your visitors will not mind having waited for it, but if it isn't helpful to them, they won't appreciate it!
Animations fall into several different catagories, and you will find buttons at the bottom of this page that will show you an example of various types of animation.
Simple animations can be produced as animated .GIF files, and these require no additional plugin support for the browser. They tend to make quite large files and so are best used for very small animations such as a rotating 'NEW' flash.
More complex animation effects can be created using Java Applets. Again, these require no additional support, but many users will have Java support disabled in their browser because there are security implications with having Java enabled. Most Corporate networks will have Java disabled.
Full animation can be achieved using Windows .AVI format files, but these usually cannot be viewd on an Apple Macintosh or Unix machine. Apple's Quicktime Movie files (.MOV) can be viewed on most platforms using the Apple Quicktime browser plugin, but there is no guarantee that your visitors will have this plugin installed in their browser. Both .AVI and .MOV files tend to be very large and yake a long time to download.
Streaming video allows movies to begin to play as soon as they start downloading. The file format requires that a suitable plugin be installed in the browser, and streamed video also requires support on the web serever. Not all ISP's provide this support.
Macromedia's Shockwave files provide good animation effects for relatively small file sizes, but they do require the Flash plugin for the browser. Current browsers ship with the Flash plugin. Whole web sites can be constructed entirely in Flash format and offer excellent interactivity for the user.
Layer animation allows areas to be moved around the web page, but only works on Netscape 4 and Internet Explorer 4 and later browsers.
An up-and-coming technology called Scaleable Vector Graphics (SVG) will allow some very exotic animation effects with small file size, and is set to revolutionise the appearance of web pages. Future browsers will support this format without plugins, but at present SVG requires plugin support and very few people will have the SVG plugin.