There are many drugs which are not legally available to the public. Generally, if we can't buy it "off the shelf" or with a medical perception , then its likely illegal to possess.
Possession can occur in essentially 2 situations:
1. Actually having a drug on your person;
2. When you know that a drug is in a place, and you consent to that drug being there, and you have some control over the place where that drug is.
Examples: Have a small bag of cocaine in your wallet or being in an apartment with a "ball/line" of cocaine on the table.