Ukuleles may be tuned in several different ways. In Sweden and some other parts of Europe the D-tuning (A-D-F#-B) is the most common. In Hawaii and on mainland USA the C-tuning (G-C-E-A) is the norm. A variant of C-tuning called lowG seems to be getting more and more popular in the USA. This tuning variant is not reentrant and the G-string is tuned an octave lower, which results in a fuller sound. The G-string has to be exchanged for a thicker string than normal to obtain a good sound with this setup.
Do not confuse lowG-tuning with normal G-tuning. The latter tuning variant coresponds to the four lowest strings of a classical guitar (D-G-B-E). Baritone-ukuleles are usually tuned in this way.
Below is a table over the most common tuning variants. The first string is the string closest to your toes.