Ritalin, aka Methylphenidate, is a CNS Stimulant, and as stimulants go, somewhere between caffeine and amphetamine. MPD is slightly more effective than ephedrine (But without the extremely high body load of caffeine or ephedrine.)

Ritalin is usually prescribed for ADD or ADHD, because of the interesting fact that in a notable percentage of the population, stimulants increases the ability for one to concentrate for a long period. Commonly prescribed when Methylphenidate is deemed ineffective is Dexedrine or Adderall, both amphetamines.

Being stimulants, they remove desire for sleep and/or food for a time period. When one has a great deal of work to do in a short period of time, this can seemingly be a godsend-- as indicated by people who Crush up Ritalin and insuffilate it for its amphetamine-like effects. Remember, though, that all stimulants are hard on your heart and nervous system, and that regular use can make you want and need more. Drugs are bad, but the high school kid quaffing a pot of coffee to finish an essay, or the doctoral candidate half strung out on meth are unlikely to care.

Alas.

By the way, if you, or anyone you know, uses Ritalin to study for tests, you might want to have a look at the psychological phenomenon known as State Dependent Learning.