The sound of the bass is in the strings
Bass strings usually come with a coating. The coating on the bass strings or even the guitar strings essentially tell you what sound the bass will produce when it is plugged into an amplifier.
Bass guitar strings are sometimes thicker or thinner as well, so if you want to groove like Robert Trujillo or if you want to slap the bass like they do in funk then you'll find a huge range of guitar and bass strings on eBay.
The Middle
Underneath the coating bass strings usually, have a core that is made out of steel. The core is the middle of the string and the windings go around it. The bass strings that you choose will either have a core of round and hex, however it is possible to find other types.
Round core bass guitar strings are very round and these produce a vintage tone that is fatter and much more flexible. Hex strings hold onto the winding more, meaning that you get a much better and brighter sound while also getting more consistency with your playing.
Windings
Acoustic guitar strings are very different to electric bass or guitar strings. The gauge determines how thick the strings are and the most common gauge is 45-105. The 45 is the high string and the 105 is the low string. 45-105 bass strings are also available in long scale. Bass guitar strings are available from brands such as D'Addario, Hefner, Warwick and Fender.
Roundwound are the most popular and are used for rock, country and jazz. Flatwound strings tend to go on fretless bases. Tapewound strings are when nylon is spun onto a roundwound string, giving the string a shorter life but much more of a thud, or punch when playing. The bass strings that you choose will depend on the type of music you want to play and your general playing style.