The word "stimulant" can refer to any type of drug that increases activity in the central nervous system or sympathetic nervous system, ranging from caffine to methamphetamine.
The most common recreational stimulants are commonly called "uppers" and include ADHD medications (adderall, ritalin, etc), amphetamines, cocaine, and ecstacy.
The more addictive stimulants generally give the user a sense of euphoria that might include alertness, possible twitching and an overall sense of well-being.
Nicotine, the addictive subtance in tobacco, is a well known stimulant but can also be sedating in higher doses.
E: Yo, i got a lot of fuckin work to do tonight, you know anyone that can sell me an upper or any stimulants in general?
K: Yeah, joe just got his adderall prescription refilled.
E: alrightt sick ill just crush up and snort a couple of those, peace