When I went through my original (bachelor) degree program about 10 years ago, my classes were all classroom courses. My current (bachelor) program is almost entirely online. Therefore, I have a fairly decent handle on the differences. Online classes are convenient in terms of logistics - you don't have to worry about driving anywhere and most of the time you can complete your assignments at 2 AM if that's what works for you. However, most of my online classes have been more difficult than their classroom equivalents. There have been exceptions - I had to take two classes with the same prof, and she completely slouched through them; I could have turned in "See Spot Run" for my assignments and received an A - but for the most part, the profs design them to push students.
In addition, online classes require a great deal of individual commitment. There's no one to nag you about reading assignments or deliver a lecture to you; you have to read the material, watch the lecture, take part in online discussions in a meaningful way (as another poster noted, profs rarely accept the "I agree" posts as valid for points), and complete your assignments on your own, amid whatever chaotic stuff may be happening around you at home. You basically have to set up an office or "quiet zone" and ensure that everyone respects it during your study time.
So before you do this, ask yourself if you're sufficiently self-motivated and disciplined to do an online class. They work for some people and they're awful for others - it just has to do with your personality and individual learning style. Don't count on your online course being "easier" than the classroom equivalent, because in all likelihood it won't be; in fact, it may be a little tougher. Personally, I love online classes and I've learned (and retained) a great deal from mine. I enjoyed my classroom courses too, but in online classes, I'm much more confident about contributing. Many people seem to feel the same way; discussions in my online classes have been a lot more lively than my classroom courses, because they aren't dominated by the same three or four people each time. On the other hand, it can be difficult to really connect with the professors in an online class; I've only had a couple so far that I've really gotten to know. And as I'm sure you're aware, summer classes always condense things, so you'll have to be very careful to stay motivated about keeping up with everything. That can be tough when it's only the little voice in the back of your head nagging you to study, as opposed to attending lecture and taking notes each day.
I hope that information helps. Good luck with your class, however you take it!