The ukulele was invented in Honolulu around 1880 by Portuguese migrants from the Azores and Madeira Islands. It evolved out of the Portuguese Braguinha but its modern form was created in Hawaii. It is essentially a high tuned 4-string classical guitar. 

The ukulele is tuned G, C, E, and A. If you capo the 1st four strings of a guitar at the fourth fret, you get G, C, E, and A low to high. One difference is that the uke in standard tuning has an octave higher-pitched G string (string 4) -- this tuning has been enthusiastically sung for well over a hundred years as the ditty "MY dog has fleas!", despite the unsavory topic of the phrase. 

It is possible also to tune ukuleles with a low-pitched G string by simply changing out string 4 and replacing it with a 4th classical guitar string, making this tuning identical to the first four strings of the guitar at the 5th fret. Listen to the strummed uke on "Over the Rainbow" to hear this low G tuning. Mostly, tenor-sized ukuleles are chosen for the low G tuning. String sets for both low and high G tunings are available at Bounty Music. The Ukulele is a quintessential sound of Hawaii .... Aloha!