With launch of hugely successful new mobile platforms iOS and Android and their respective app markets, Apple's App Store and Google's Android Market, mobile apps have become a major phenomenon. Gone are the days when most of the downloadable apps were simple remakes of old classic arcade games for hardcore gamers or add-ons for business software targeted at e.g. maintenance personnel working on the field.
Today mobile apps are one of the best new ways for brands and companies in consumer business to keep their customers loyal and constantly engaged with them. Unfortunately developing a mobile app can also be time consuming and costly - especially if there is a need to target as wide audience as possible:
The stellar success of Android-based devices has changed the mobile app landscape so that in almost all cases both iOS and Android versions of the same application are required. Usually that means at least double development effort and some times even more if the whole range of different Android devices need to be supported. In case there is a business need to support other platforms as Windows Phone or Symbian, too, the required development effort might easily be 200-300% more.
Leveraging web technologies such as HTML(5), CSS and JavaScript is one potential cure for the problem of supporting multiple platforms. Instead of developing several incompatible native versions of the same app it is possible to develop a single web application that works directly or can be ported with very little effort to all major mobile device families.
Of course as mobile web apps are run within mobile device's web browser it causes significant overhead. Thus, performance-wise (speed, memory consumption) native apps are a better choice. Still, in many cases the performance hit is totally acceptable when considering the benefits - e.g. shorter development cycles and smaller development budgets. A word of warning, though: The more the planned app relies on platform specific features (e.g. accelerometer or geolocation interfaces or access to phone's address book), the less suitable web technologies currently are.
Another key advantage of leveraging the web technologies in mobile app development is the very low learning curve. Instead of learning a new platform-specific programming language and taking grasp of the needed tools, an experienced web developer can become highly productive mobile app developer almost overnight.
Still, it must be emphasized that even though it is easier to get started with developing mobile apps using the web technologies than by learning how to develop native iOS or Android applications, developing well-functioning and optimized mobile web apps requires deep knowledge and lots of experience in HTML(5), CSS and JavaScript. In case you have any doubts, I suggest you consider hiring a professional. ;-)