Yes, you can build mobile apps in HTML. You do not need to use Java or c-sharp (unless you want a complicated app). There are easier ways for the novice programmer. This is good as my students are not really programmers at all, we are designers:) So you can build standard native apps in HTML. My favorite tool for doing this is PhoneGap. PhoneGap is an opensource software package owned by Adobe. Here is a link to phonegap:
Here is a review of PhoneGap and my thoughts on using it in the classroom: http://raypastore.com/wordpress/2012/05/phonegap-building-an-app-and-classroom-use/
Now, what is the advantage of using HTML to create a mobile app? One, its pretty easy and many people already know HTML. You can even use dreamweaver to make the app – although be careful because dreamweaver creates some bad code that might not work well on the mobile device. I really suggest hand coding in HTML5 all mobile apps. But the real advantage is that the apps will work on all mobile devices! You can publish to Apple, Google, MS, and Blackberry. Now in order to do so you need to be a developer for each and publish through them, so for Android its Eclipse and Apple is xcode – and as you read in my review blog post above, that is kind of difficult for the average user. However, for the developer, this is an awesome way to create one app that works on all devices.
One thing I do want to mention here and make sure this point is clear: HTML5 apps are simply not as powerful as apps developed in C-Sharp, Java, or Flash. HTML5 just doesnt have the power. So for simple apps, HTML5 is a great option and the best in my opinion. For a more complicated app that access a database and requires a lot of functionality, I would go a different route.
Just a quick note – there are other options besides phonegap, such as: titatium and genexus. I chose phonegap because they really seem to be the most popular, are opensourced, and now that Adobe owns them, I have a feeling they will be integrated with the next version of dreamweaver and will really own the HTML5 mobile market.