For some people Sativa is over-stimulating and can cause paranoia, but at the same time Indica might just be more sedating than they are looking for, for people that don't fit on either ends of the spectrum, there is Hybrid strains.
Best of Both Worlds
Hybrid strains occur when breeders, cross two different plant to create a new strain; like the west coast's beloved Blue Dream which is a cross of Haze (a Sativa) and Blueberry (an Indica).
By crossing these strains they have decreased the stimulating effects of the Haze and the overall sedating feeling of Blueberry, landing in the middle, giving it a mild head high while also allowing for a nice body high to relax the user.
Hybrid Strains are for people that are looking for the best of both worlds, a nice effective head buzz while they enjoy a relaxing and comforting body high.