Wrap It Up

Beyond the more strings and odd spacing, guitar strings have one major difference: Two of the strings aren't wrapped. Bass strings are all the same, only differing in gauge [or size]. While all bass strings are wrapped in metal [or wound, like Flat would, Round wound  et al] only the four largest string on a guitar are like that.

Now I've only ever noticed Lula strings [so this is being said out of damn near pure ignorance] but the tiniest two strings are just wire; what is the core of the other strings.