I found a really interesting and accessible site explaining how various drugs cause their users to get high:
http://learn.genetics.utah.edu/units/addiction/drugs/mouse.cfm
One thing that I thought was interesting is that almost all of the drugs mentioned affected users by causing your brain to flood itself with neurotransmitters, namely
dopamine. Many of the drugs work by inhibiting the reuptake of these neurotransmitters much like antidepressants work, which surprised me. I guess we often forget that things like antidepressants and ritalin are just less intense versions of things like ecstasy and methamphetamine.