Strictly speaking, speed refers to amphetamine or methamphetamine.
It's most likely the clandestinely produced pills (ie, look like E pills) sold as being "speed" would contain methamphetamine (though they could be BZP, amphetamine, or some random RC - just like E pills, you have no guarantee of what they are). Not very common in the US, except when sold as E. I believe this is what the OP is thinking of.
Diverted perscription "speed" would be adderall (3:1 d-amp : l-amp), dexadrine (d-amp) or desoxyn (d-meth), or from sketchier individuals, any other perscription stimulant, like ritalin and the like. And these would of course look like professionally made prescription pills, and would likely not be sold as "speed", but as whatever pharm they are ("addies", "dexies", etc)
Powder sold as "speed" is usually meth cut to hell with inert ingredients.