Here is a quote from Hilary Black of the British Columbia Compassion Society:

"Indica and Sativa are the two main varieties of the cannabis plant used as medicine. There are many strains that are crosses of those two varieties. Within each of those varieties and crosses there are a huge number of individual strains, each with a different cannabinoid profile and effect. According to anecdotal evidence, the Indica strains are a relaxant, effective for anxiety, pain, nausea, appetite stimulation, sleep, muscle spasms and tremors, among other symptoms. The Sativa strains are more of a stimulant, effective in appetite stimulation, relieving depression, migraines, pain and nausea. We are now aware of specific strains that are effective for specific conditions and symptoms."