They vary by state. Get online and fine the ones that are sanctioned by your state (and you won't have any trouble, trust me) and pick one that looks like it'll be amusing. The state will have very specific standards, so they're all going to be the same length, ask the same questions, show the same videos, and cost the same. The only thing that'll be different is how they'll be presented, and for your own sanity, I'd suggest that you find one that looks like it'll be funny.
Personally, I'd bite the bullet and do it in person. You should be able to find a defensive driving course offered by a local comedy club. It'll be the same 6 or 8 hours, but they'll occasionally make you laugh.
And though you pay online, you'll still have to mail in your paperwork. In Texas it's about 5 different forms.