Sativas are usually associated with more of an energizing, creative high for daytime use, some also feel that sativas help with depression, appetite loss, and nausea.
Indicas are more commonly used in the evening to wind down and give a more sedating (couch lock) effect, these are linked to pain relief and can also stimulate appetite.
Hybrid strains are a mixture of the two which helps to balance the effects, this gives you the relaxation of Indica but thanks to the sativa influence you can still be productive and energetic.