Web app:
Web app is another name for website specially build, so that it runs properly on smart phones.Its UI is designed with web standard technologies.Web app is build on url,it is not installed on phone or is not available at Android Market.

Native App:
It is app,which is installed on mobile phone and is available at Android Market,they have access to the hardware (speakers, accelerometer, camera, etc.) . Once installed we can use it without installing again and again.