All drugs are chemicals. Depending on their chemical structures will depend on how drugs can affect the body and the mind of the user and in different drugs act in different ways. In fact, some drugs such as LSD can even change a person's body and the way they view the world for many years after the person has stopped taking drugs, maybe even permanently.
Depending on the drug, they enter the human body in a number of ways, ranging from direct injection into the bloodstream, inhalation, and orally. Some drugs of the same kind can have a different impact on how the drug affects the person, depending on how they were administered. For example: because injecting puts the drug straight into the bloodstream, the effect is felt almost immediately; while swallowing requires the substance to pass through the stomach giving a slower effect.
Nearly all street drugs directly or indirectly target the brain's reward system by swamping the organ with dopamine. Dopamine is a neurotransmitter that regulates movement, emotion, cognition, motivation, and feelings of pleasure. Once a street drug enters the brain, they can change the brain's function and how the brain goes about doing its job. It is these changes of how the brain works that eventually lead to compulsive drug use, the hallmark of dependency.