Botanically speaking, the terms indica and sativa describe physical differences between marijuana plants. The terms describe marijuana's effects on the body.
When you think of indica, think of "in-da-couch", because that's where your (gr)ass is about to be, buddy. Indicas bring on relaxation, lethargy, and hunger. When you think of sativa, just think the opposite of "in-da-couch", whatever that means to you. Sativas inspire energy, creativity, and sociability. If you plan to wake-and-bake (smoke weed in the morning), smoke a sativa. A hybrid, then, is a marijuana strain that is slightly social but slightly sleepy, too.
It's a spectrum, but every type of marijuana falls broadly into one of these three categories, so choose wisely. Armed with this knowledge, you are prepared to answer the next logical question: how should I get stoned?