Web based, web launched, AJAX, Java, OPC UA - these terms are commonly thrown around along with HMI, SCADA, and even DCS these days. What's the big deal? More importantly, what's the point and what does it mean for you? The common thread is ubiquity. Yes, I'll say it again, ubiquity. I don't know why there's not a more common word with the same meaning - to be, or appear to be, everywhere at once. It's the perfect word to describe the Internet. So when somebody says, "Web based", think, "That means I can access it anywhere". That means it's firewall and VPN friendly. Nobody said anything about web browsers, static HTML, http, or the likes! Web applications, particularly Java and Macromedia Flash, run and feel just like local applications. They support multimedia, run constantly, and can initiate and receive updates without "refreshing". They're locally running programs with the huge benefit of not requiring a traditional "installation" process!
So where does SCADA come in? An important aspect of a modern SCADA system is to be able to get detailed realtime and historical process information. For most production managers, this is the most important data for their day to day work. It's like checking stock quotes - a 30 second glance should give you an accurate summary and a warm fuzzy feeling that you know what's going on. If something demands action you want to know. How much sense does it make to go to your stock broker every time you want a quick update? Should you fly out to Wall Street? No, it's valuable for you to be able to easily pull this up from your office desk, or home. What does this have to do with SCADA? Same principal applies. Who wants to go to the control room or plant floor every half hour? Ideally, you should get a portal or summary page that provides a high level summary with reports. The idea is that you have access to the same underlying data, but formatted as useful information to you.
What actually happens in an organization that provides "frictionless" data access to their core process is that everyone comes up with separate requirements. QA wants summaries, management gets reports, maintenance looks at long term statistics, etc. All that it really takes is a system that can be run anywhere and easily expanded - "easily" referring to without additional licensing pain.