Methamphetamine is amphetamine with an extra methyl group. Like amphetamine it has dextro and levo isomers. Dextro-Methamphetamine is more potent and more recreational.
The freebase of meth is actually an oil, what you have on the street is methamphetamine hcl (or supposed to be, its really a mix of a lot of shit thanks to the government and their stupid laws). That said a freebase (ex: crack cocaine) is the pure base chemical, not attached to a 'salt' (Ex: HCl).
Unlike cocaine, meth can be effeciently smoked as a salt but there are still two forms to be found on the street. One is the powdered meth and the other is 'crystal' or 'ice'.