they are different categories of drugs and a drug can fit into more than one category and these aren't the only categories. depressants are drugs that make you tired or drowsy and make you not want to engage in physical activity like weed, alcohol or heroine. Stimulants are drugs that make you energised and are often very hard or impossible to sleep on, such as meth, coke or any kind of "speed". Hallucinogens are sometimes referred to as psycadelics. These drugs confuse the neural pathways, altering your senses and changing your thought process and perception of all things. Many experience it as an entirely different realm of reality. LSD, mushrooms, peyote, DXM, DMT and salvia just to name a few.