Plug-Ins are downloadable browser-components that enhance the feature set of a given browser. The most well-known plug-in is Macromedia Flash with the Shockwave plug-in. Technologies like Flash are a black hole from an information point of view. The data is simply not accessible on the client, can't be indexed in meaningful ways be search engines, it doesn't adapt to browser settings like font size. There is rarely a good use for plug-ins, and mostly they're plain annoying and barriers put between user and destination. Plug-ins are media specific and not flexible, and have higher maintenance costs for even simple things like a text change.
- If possible, do it with HTML
- If not possible with HTML alone, add CSS
- If not possible with HTML + CSS, do it with HTML + CSS + JS (DHTML)
- If not possible with DHTML, consider Java
- If not possible with Java (or too slow, or too complex), consider a plug-in, but know that you may lose audience
The only good use of Flash is to embed movies; if the plug-in itself is the content, as opposed to the navigation, and no meaningful text-alternative can be provided.