Aquila strings are one of the most popular brands of ukulele strings around, if not the most popular.
They are made in Italy of their own, special Nylgut material. Nylgut is a synthetic material that is intended to combine the best qualities of nylon and gut strings (hence the name). The strings are easy to play and have a warm tone to them.
There's plenty of debate on the ukulele forums on this subject, different people prefer different strings. Overall, it seems like Aquila are winning the string wars. Even though I'm a Worth man at heart, I regularly and quite happily use Aquilas.