What is the significance of the auto generated appcompat_v7 project/fragment.xml file

So I decided that I wanted to update a few android applications that was working on. I started using the ADT rather then going through installing the plugins in eclipse as used to have to do. I realized that unlike before, when I create a new project, there is an auto generated project named appcompat_v7 (Theres a new one created every time I create a new android application project). Additionally, there is an auto generated fragment file (ex. fragment_main.xml) every time I create a new activity. How are these files used by the project? When editing an activity layout, do I just edit activity.xml file, or do I need to do something with the fragment_activity.xml file?