I think there are mainly two sorts of interactivity in relation to the web:
(1) interactivity between the user and the web page (or actually the server hosting the web site)
(2) interactivity between different users across a web site.

Examples of (1) is e.g. that you can submit a form or click on a link and get the server to react differently depending on what values you enter in the form or what link you click on (and the specific data conveyed by the link). E.g. you can get the server to store certain data in a database table or a file, delete or manipulate values in a database table or file, send you to a specific web page with content customized to the data you sent in the form, etc. This sort of interactivity is essential for processing form submissions, doing blogs, discussion forums, web mail, shopping carts and e-commerce, handling member-customized areas on membership sites (e.g. the personal dashboard which each member has at proz.com), etc.

Examples of (2) - when users interact across the web - are e.g. discussion forums, blogs, web mail, etc.

Most - if not all - interactivity of type (2), i.e. btw. users, presupposes interactivity of type (1), i.e. btw. user and web server. Also, the concrete value of interactivity of type (1), and the reason why we appreciate it, to a large extent has to do with its capacity to enable interactivity of type (2).

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Note added at 4 hrs (2008-12-08 02:20:14 GMT)
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This sentence: "Med XXX kan du skapa interaktivitet på webben utan kunskap om tekniken. " - The point is probably that a person/web site administrator can use XXX to have "cool" things on his web site, like forms, discussion forums, shopping carts, etc. - without needing to know how to program these things on his own. (Common scripting languages used to make web pages interactive are PHP, Python, Perl, etc.)

So-called Content Management Systems often used by non-programmers to quickly set up web sites with basic interactivity without needing to learn scripting/programming. I guess your XXX product is similar to content management systems.