Why does index.php file get invoked when the clicking on link http:///groups

I'm trying to understand the template file hierarchy - when I click on the 'groups' link, the current groups are listed. When I hover over the 'groups' link I see that the url points to a groups url, as in http://website/groups. Which leads me to think that the groups.php template file will be invoked, but instead the index.php file gets invoked. I wanted to know how the translation of http://website/groups invoked index.php instead of groups.php, Please advise. Note that the index.php file being invoked is the one under the themes/<active-theme>/groups directory.