Ukulele strings come in 4 different sizes, for soprano, concert, tenor and baritone ukes. They can also be made from a number of different materials, most commonly, gut, nylon and fluorocarbon. Gut strings are the original materials, then nylon was introduced as a cheaper, consistent alternative. They sounded great, albeit a little differen to gut. Fluorocarbon strings (fishing line) have now become quite popular as well. Here are some offerings from Rotosound, D'Addario and Alice.