A neurotransmitter is a chemical that is used to send messages from one neuron (a.k.a nerve cells) to another. When a neuron receives an action potential, an influx between negative and positive ionic discharge, neurotransmitters are released from vesicles into the synapse where it diffuses to another neuron. Receptors on another neuron pick up the message and the same process occurs over and over again until it reaches the brain. During this process, the message may travel through millions of cells but it happens almost instantaneously.
There are specific neurotransmitters related to a certain traits or actions. For example, serotonin/emotions, dopamine/desires, nor epinephrine/arousal. The use of drugs and other subtances causes an abnormal releases of these neurotransmitters which causes an abnormal feeling that could lead to increased craving. Therefore forming an addiction to the substance or action.