Building mobile application with PhoneGap also known as building a hybrid application. What is a hybrid application? It is an application that runs with browser components, but could be able to utilize the phone functionality (such as camera, geolocation, notification and etc). Hybrid application is downloadable through marketplace. If you want to build native application for all different platform, you need to use C# for Windows Phone, Objective-C for iPhone and Java for Android. However, building hybrid application, you only need to take care of HTML, CSS and JavaScript.

Why PhoneGap?

• Take advantage of HTML5 and CSS3
• Use JavaScript to write your code
• Open source & FREE
• Access Native Features
• Deploy your app to Multiple Platforms
• Take advantage of PhoneGap Build
• Add PhoneGap Plugins to your project
• Use Tools from the community
• Get help from the growing Community

You can obtain more details and resources about PhoneGap from here and here.