With server side techniques, the user enters information on your page and that information is sent back to the server. The server takes that information and then "builds" a custom page that is sent back to your browser. The main advantage of this type of interactivity is that for complex operations, the higher powered server can perform them much quicker than doing them on the client machine. The main disadvantage is that you have to send information over the data lines, usually the slowest part of the Internet experience. The are lots of technologies you can use to implement Server Side Interactivity. Some of the more popular ones include:

• PERL - The granddaddy of them all.
• Cold Fusion - The leader in a new breed of server side scripting languages.
• PHP - An interesting, home-grown, open source option which combines features of many others
• JSP - Java Server Pages. A new and promising scripting implementation
• ASP - The ubiquitous Microsoft foray into server side scripting.
• Many others

Here is a ZDNet article that describes 4 of the languages mentioned above. 4 Scripting Languages

Client Side Interactivity differs from Server Side in that all of the computing goes on at your client, that is to say, your browser. You can still do quite a bit of interaction using Client Side techniques, and can even maintain information between visits using cookies. By far the most common Client Side method is JavaScript (newly named ECMAScript). JavaScript is a language created by Netscape, and has very little to do with Java other than marketing hype. Other languages to use include Microsoft's ActiveX and PerlScript. ActiveX is available only on Internet Explorer, and PerlScript is too new to have taken hold yet. With client side, you alter the appearance of the page without sending information back to the server. This allows you to avoid the slow transmission times, but does require the users' machine to have a little more umph to get the job done. While JavaScript will work on most browsers, the differences in implementation have caused web developers nightmares for years. With the introduction of the standardized DOM specification, there is a chance to eliminate many of the browser specific problems that have been around for awhile. However, there is yet to be a browser that contains all of the standardized DOM features, although IE5.5 and Mozilla have come close.

As I mentioned before, one of the most common interactive things to do on a web page is send emails back to the site owner. Usually this is done with as a server side program, and sent from the server. But you can get around that with a simple form and using the mailto: action. A brief explanation here should give you some insite on how to get that working.