There are two main flavors of bass strings: roundwound and flatwound. All wound strings are made by wrapping layers of wire around a core wire. Roundwound strings use a round wire as the wrap, flatwound strings use a flat ribbon wire.
Roundwound strings deliver brighter sounds, but can emphasize squeaks. Flatwound strings have a duller sound, with less extra noise, and tend to keep a more consistent tone longer.