What's the difference between Web browser and APP if we surf to a site with both version?

Do you know What's the difference between Web browser and APP if we surf to a site with both version?

For example, the CNN has its APP for Andorid. However, if you do not want to install APP, you stil can surf the website by your browser of your mobile or tablet.

Do you know if both of ways present the layout by XML(HTML)? Is there a way (tool) to tranfer one to another easily? What's the advantage if the website provides its specific APP besides original web server?

Thank you~