There are 2 main types for electric basses, roundwounds and flatwounds. Roundwounds have a more attacking, zingy tone to them whereas flatwounds are a little darker. Roundwounds are more popular for fretted basses and flats more popular for fretlesses as they don't gouge the fingerboard as much. You can also get strigns in between round and flat, often called half-round or ground-wound. The material that the strings are made out of also affects the tone, with stainless steel a little brighter than nickel. Copper or phosphor bronze strings are only used in acoustic basses as they have poor magnetic qualities.