Well first off there are three types Sativa, Indica, and Ruderalis. Ruderalis is basically a dead strain. So the main types of marijuana you will get is either sativa or indica. Sativa is considered a head high and Indica is considered a body high.
Sativa
- tall, average 4 - 15 feet
- long internodes between branches, 3 - 6 inches
- thin, long and pointy leaves with no markings or patterns
- usually 6 and 12 blades per leaf
Indica
- small, average between 6 inches to 4 feet
- short internodes between branches, 3 inches or less
- wide, short and rounded leaves with marble-like patterns
- usually between 3 - 5 blades per leaf
Ruderalis
- small, average between 6 inches to 4 feet
- very short internodes with much branches
- small and thick
- usually between 4 and 6 blades
Most commonly found today is only sativa or indica. Ruderalis is less potent and not seen often.
These are the different types of species of marijuana, but there are different types of highs. That is why different marijuana helps only certain sicknesses, but I'm not to familiar with that aspect.