If you're buying weed at the dispensary or looking up online what your dealer just claims he gave you, someone is going to be breaking up pot into three different categories: indica, sativa, and hybrid.
Indica strains is the weed you smoke to relax and watch TV or right before you knock out for the night. Sativa strands on the other hand are the exact opposite. Sativas will probably keep you awake and good for parties or doing something creative like painting/making music. Hybrids are both ndica and sativa, so you really never know what you're going to get.
Someone who would be picky about which weed to smoke is a medical patient using marijuana to treat their symptoms and/or conditions. For example, someone battling depression would use a sativa, and someone battling insomnia would use an indica to fall asleep at night. As for the hybrid strains, which have a higher THC level than most others, are used for those suffering from seizures, anxiety, pain, and a few other conditions.
The breaking up of weed into different categories started back in the 18th century. This is when people began to realize the differences between buds. Indicas are believed to be originally from the Hindu Kush region near Afghanistan, and sativas were found in Asia closer to the equator.
Check out this chart which compares the two for you via Leafly: