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 finger squeaks. Flatwound strings have a duller sound, with less extra noise and tend to keep a more consistent tone longer.

Bonus tip: 
You can rejuvenate your roundwound strings by boiling them in vinegar or mild detergent solutions. It really works, and here's why: As you play your bass, dirt, oil, and grime from your fingers attach themselves to the strings, between the windings. This build-up dampens the vibrations of the strings, resulting in your strings sounding dull or "dead". Boiling the strings in vinegar or a detergent solution helps to loosen and remove the build-up, rejuvenating your strings!