The law prohibits the possession of a controlled substance - such as marijuana, cocaine, and methamphetamine. Possession has a specific meaning under the law; not surprisingly, that definition is not the same one used in everday life.
In Texas law possession means "care, custody or control" over something. Custody is fairly straightforward, and means what you would think it means. The other two terms however are somewhat broader than the normal understanding. This is an important issue when drugs are found somewhere other than on your person. The most common situations are drugs found in a car, or in a house.
To complicate the situation even more, there is a concept called "joint possession". That means that two or more people can possess the same thing.
So how does this all work? Suppose you're the passenger in a car and drugs are found in the glove compartment. Since the driver has care and control over the car, he (or she) would probably be charged. The passenger might get off, unless there was something tying you to the drugs. The situation might be different if the drugs are found somewhere else, like under the seat, or on the seat. The issue then become whether you had control over the drugs.
It's not unusual in such cases for the police to arrest everyone in the car and let the prosecutor decide who should be charged. They only need probable cause to arrest you, which is usally present whenever drugs are found.
The same situation can arise when drugs are found in a house. If it's your house, you are probably going to responsible for everything inside, since the house is under your control. If you're just visiting, it usually depends on where the drugs are found, and how much there is.
Where the drugs are found is important. When they are out in the open the police may claiim the drugs are under your control. However, it's not enough to prove you knew the drugs were there; that's not enough to convict. There must be something to more to establish that you not only knew they were there, but that you had care or control of them.
Another important issue is the amount of drugs found. It's a lot easier to claim you didn't know about the joint under the seat, than 50 pounds of marijuana in the back seat.
You probably figured out there are no easy answers in cases like this. Guilt or innocence depends on the specific facts of the case. For this reason it is extremely important to obtain a lawyer who is has experience with drug cases, and fully understands the law of possession.