Students usually clear out of Grounds right after finals, leaving quite an empty campus. However, I have quite a few friends staying in Charlottesville this summer for the purpose of taking classes. Personally, I detest using summertime for more classes. It's a time to relax, to delve into your hobbies and interests, and to recharge. Not time for a continuation of school! But I understand their motives.
Summer school at UVA --> how common? Mm.. not that common but still common enough to be not rare. Not that many students take classes, but some still do. Now, I've heard rumors that summer classes are easier, that they grade more easily. They are, as far as I know, rumors. I would think it's easier in the sense that there are less distractions around. Most of your friends have gone home, there's not that much happening, so you really only have your books and notes for entertainment.
But summer in general on Grounds is quite bare. Most have gone home or elsewhere for internships and jobs, yet others are studying abroad or doing research. It's not a completely dead town though, there are still townies and Charlottesville still does have a life outside of the college students. Our University Programs Council still puts on programs, and some fraternities still host some parties.