What they do: in a nutshell, they improve insulin sensitivity. Better insulin sensitivity means that your muscles soak up more nutrients, your blood sugar isn't up and down all over the place, you're not as hungry, and your risk for diabetes later in life is diminished.
What's the difference between the two: r-ALA is the naturally occurring form, while racemic ALA is a mixture of r-ALA and s-ALA. R-ALA is better at improving insulin senstivity, but supposedly racemic ALA is a better antioxidant.
When and how do you take them: post-workout, or with meals, especially meals that will cause a large release of insulin.
Optimal dose: 100mg of r-ALA per 30-50 grams of carbs. The dose for racemic ALA is harder to approximate, because there's no telling the porportions of how much r- and s-ALA might be in the ALA mixture. 300-500mg of racemic ALA with a meal is a good ballpark figure.
Hope this helps.