Ritalin is a stimulant, however it is not technically "speed" (amphetamine/methamphetamine).
Amphetamines promote the release of dopamine and norepinepherine from the presynaptic terminal. This increases the concentration of these neurotransmitters in the synaptic cleft.
Cocaine has the same net effect, however it is accomplished by blocking the 12-helix dopamine reuptake pump on the presynaptic nerve.
Methylphenidate's primary action, like cocaine, is to block the dopamine reuptake pump. It also may stimulate the release of dopamine and norepinepherine from the presynaptic nerves--as do amphetamines.
The net effect of all of these drugs is to increase the availability of catecholamines (dopamine, norepinepherine) to stimulate the postsynaptic receptors. Studies have shown that cocaine addicts have had a difficult time distinguishing between stimulant drugs when they are administered intravenously.
However, the doses that are used for treating ADD/ADHD are not the same as those used by abusers to obtain the illicit effects. At lower doses, these drugs can promote concentration and focus, depending on the individual.
-Pharmacolic