A possession charge is determined by looking at whether or not somebody had dominion and control over the drugs, which doesn’t mean that they had to have bought and paid for the drugs. It simply means that the drugs were in their presence, whether that means they were hidden in a part of their house or under their car seat.
There are two different types of possession: actual possession and constructive possession. Actual possession means possession of drugs on your person. Constructive possession means that the surrounding circumstances indicated that you had or intended to have some control over drugs.
A sale would mean exchanging a drug for something of value, which doesn’t have to be money; it could be a favor or anything of value. In California, there is no differing penalty between sale, distribution and providing a drug to somebody else. They are often bundled into the same law and are dealt with equally. The circumstances just indicate the specific difference between selling, distributing and providing.