A Sativa typically grows quite taller and oftentimes the leaves are much thinner and further separated, whereas Indica grows shorter and more 'bush' like.
As far as efficacy, Indica is more of a body stone; very relaxing. Good for pain relief and for sleeping. Indica is typically what gives one 'couch lock'. Sativa is more of an 'upper'; promotes creativity and helps with energy. Many people I know wake-and-bake with Sativa so they can go about their day, then use Indica at night.
I don't rarely smoke, but I use Indica edibles to soothe my Restless Leg Syndrome and other sleeping issues. I like Sativa periodically as it spurs my writing creativeness, and it's great for having laughs with friends.
Also, Sativa typically takes longer to flower (strain dependent) than Indica does.
-spek