There are three strain groups but only two are good to smoke.

Sativa strain is known to give an "up / headdy / trippy" high.
The plants normally grow taller, and take longer to grow.
The leaves are more slender, skinnier.

Indica strain is known to give a "couch lock" high, a sedative effect.
The plant grows quicker than sativas.
The fingers of the leaves are fatter.

Thats what i know to seperate the two, welcome