Stimulants are all psychoactive drugs that affect wakefulness, focus, and resistance to fatigue in a positive manner. Whether the increase in functionality is actual or merely perceived depends on a wide variety of factors, and may be a functional illusion to an extent, meaning that simply because the user feels like they're more capable they perform better. They are often referenced by the term 'uppers' due to their ability to provide a general or particular increase in ability, performance, function, willingness to do tasks, even ambition in some cases, and have a wide diversity of slangs committed to them, including, but not limited to : "Fire Reds, Benz, Speed, Diet Pills, uppers" etc.
Their general mode of action is at the level of the central and peripheral nervous system, but their specific mode of action varies depending on the drug, type, and group. Cocaine for example, binds to neurons and prevents the reabsorbtion of the neurotransmitter dopamine after it has been released to transmit nervous messages, leading to an increased stimulation of neighboring neurons, while amphetamine simply forces the exocytosis or release of dopamine from their storage vesicles.
It's just like the ubiquitous use of alcohol as "liquid courage", alcohol doesn't provide any actual enhancement of ability in any regard, but it can increase performance at certain tasks because normally the individual's performance would be adversely affected by anxiety, and alcohol reduces anxiety allowing the individual to perform better than they would sober. Stimulants are trickier though, because the levels of performance enhancement they're capable of is so much greater, almost universally applicable in terms of tasks, and often much easier to measure objectively.
Alcohol might seem to aid you in conversation, but conversational adeptness is a lot less tangible than a 20 page research paper in one night, 40 hours of overtime on a paycheck, driving from New York to California without stopping for anything but gasoline and cigarettes, having sex all night, not getting bored playing baseball, winning World War II, or just about anything else you might set about doing.
 Uses and why it's loved
Whether stimulants only unlock latent capacity through their psychological effects or actually increase the absolute upper limit of an individual's capability through neurological tinkering, the effects are undeniable. Fighter pilots, truckers, waitresses, college students, professional baseball players, mathematicians, and musicians have all been known to find stimulants incredibly useful and all those groups, along with many others, have a long history of stimulant use going back decades if not centuries.
We still give Dexedrine to Air Force pilots on long missions, calling them "go-pills" with the usual brash, enthusiastic obviousness of military lingo. The military doesn't dick around with shit that doesn't work unless it's a massive waste of our tax dollars, amphetamines are too cheap and comparatively boring next to laser guns and jetpacks for the military to have an interest in them if they don't actually work.
Truckers are constantly under pressure to drive as many miles as possible, because more miles means more money, so they've also been known to do the same as the Air Force, only instead of handing out the pills the government has made it illegal or at least discouraged for commercial drivers to use stimulants. It's ok to tweak in a multi-million dollar airplane, but not a Freightliner. People who work for hourly wages, especially at jobs that demand constant alertness, energy, and generally running around getting shit done like construction workers, restaurant staff, and the like also find stimulants very useful for putting in more time to get more money and maintaining the quality of work despite an environment of repetitive tasks and high pressure.
Stimulants aren't only useful for tasks that require constant focus like piloting a vehicle, hammering nails, running around waiting tables, and the like, they also have the capability to increase mental performance in just about every sense of the word. Thinking men like Sigmund Freud and Paul Erodos produced incredible work under the influence of stimulants, and likely had insights and revalations that they wouldn't have if their capacity for thought and speed of processing hadn't been increased by stimulants. Jack Kerouac, Ken Kesey, Hunter S. Thompson, Ayn Rand, whatever you might think of their work or them personally, all show a clear influence from stimulants in their writing, and like Freud and Erodos it's probable that they'd have produced less work without using stimulants. Especially Rand, there's no way someone could write a book like Atlas Shrugged otherwise. Musicians also find stimulants very useful, both for creating work and performing. The Beatles were all popping phentermine and other "go-pills" before shows through the first part of their career, Lennon being known for taking many times more than the recommended dose. Johnny Cash is going so hard in some old videos that he can't make any facial expression other than a shit eating grin or teeth clenching grimace, never sits still, and doesn't blink once. If you listen to "Ring of Fire" you can't help but think of an amphetamine comeup, and wearing all black like Johnny did is another clear indicator of stimulant use, I don't know why but it seems to fit. Cocaine is another big one for musicians, from the classic rock bands of decades prior to today's rap artists (DJ Whiteout isn't named after correction fluid), and everyone in between. Other artists of every stripe find inspiration and enhancement in stimulants, and their usage and influence on art will remain constant.
This is merely a primer, a few examples of how strong stimulants are used and the effects they can have, and how some people have made excellent use of them. Like any drugs, and possibly more so for a number of reasons, stimulants are potentially dangerous and capable of producing dependence, addiction, and psychosis. The superhuman peaks they can soar to are evenly matched to the valleys of destruction and harm, both pharmacological, psychological, and physiological that misuse can wreak. Stimulant use is about managing the downsides and maximizing the upsides, and if handled properly the problems can be reduced to an insignificant level, while the benefits can do much more for you, right then, in the real world than the esoteric and personal sort of positives associated with hallucinogen use. Everyone can use a little boost sometimes, and while pondering away at the nature of reality or society or spirituality and such is all well and good, some of us are happier when we have a paycheck loaded with overtime or a 20 page research paper instead of "inner peace". Wear all black, not tie-dye.
 Adenosine receptor antagonist
 Dopamine/noradrenaline releasers
- Cathinone and Khat