What do we mean by a 'Dynamic Site'.
A dynamic site contains elements that make your web pages more interesting and entertaining. These elements could include:
JavaScript is a client side scripting language. Because it works on the visitor's machine it operates very quickly. JavaScript is used to move objects including text and graphics around the page. It can be used to display the current time. It can be used to display changing text. It could display certain messages depending on the day of the week or the time of the day. Commonly it is used for 'rollover' effects. When you put your mouse over a link that link might change colors or cause something else on the page to change.
Animated GIF's are little cartoon like files. Sometimes it is just two frames of a small graphic that alternate back and forth. Perhaps a little star that says 'Buy Now'. The background of the star might change back and forth from yellow to red. Sometimes these gif's are used for banner ads. There might be a bright blue background with a company name. Every two seconds it switches from the company name to the phone number for the company.
Flash Files are somewhere between animated gifs and a movie file. They are very similar to power point files. The designer can manipulated text and graphics. They objects can move around, fade away, zoom in, and disappear. Entire sites can be built with Flash.
DHTML is a combination of JavaScript, Cascading Style Sheets, and HypeText Markup Language. The effects are similar to the ones mentioned above for JavaScript. There are more options with DHTML because any element on the page can be changed based on a move of the visitor's mouse.
Should you have a dynamic site? Perhaps. I would suggest keeping these dynamic elements to a miniumu.
JavaScript and DHTML can create some very cool effects. One major downside is that they do not always work. This is due to some technical reasons regarding the different browsers that people use to view your web page. I think these effects should be included if they make your site more effective. If they simple show off something 'cool' that the web designer can do, then they should not. Also question whether all your customers will like this effect. I personally find some of the effects annoying. Perhaps not the first time but after clicking through a few pages it gets a little too much.
Animated GIF' can be cute or annoying depending on the visitor's tastes. Since you want everyone to like your site, I would keep these to a minimum. They also often have a large file size, which might delay your customer from being able to see your site. Sometimes they can be effective if they fit in with the theme of your site and are relevant to your business.
Flash Files are very cool. There are some significant downsides. The files are very large and take a long time to download especially for customers that have dial up. Also not everyone has the software to display these pages properly. They can be time consuming to create and therefore expensive. They are often used by upscale and trendy businesses. If you are in the high fashion or media business then you should consider incorporating some flash. Otherwise I would skip it. One exception might be for a picture gallery. A flash version of a picture gallery can be quite effective and does not take long to create.