Just because someone knows how to roll a joint or visits a local dispensary, doesn't mean they know the all the little specificities about cannabis. One of those specificities is the difference between sativa and indica strains of marijuana. Luckily, there are people out there who can help those people out.
A user on the r/Trees subreddit (which is not a forum for fans of pine or oak) posted a handy-dandy guide to the differences between sativa and indica strains. Take a look:
A commenter on the original Reddit thread noted that most marijuana you'll find is most likely a hybrid of the two strains, and that it's actually fairly rare to run into a pure sativa or indica strain. But at least this chart will help you not sound like a complete noob the next time you're smoking up with any of your stoner buddies.