It depends.
The "standard" ukulele tuning today (for soprano, concert, and tenor ukuleles) is C tuning, in which the strings are tuned G C E A. I think it's called "C tuning" because the C is the lowest note (if you're using a high-g tuning).
Historically, though, some players preferred their ukes tuned a whole step higher. That's "D tuning," and the notes go A D F# B. I've never tried it, but it's supposed to sound brighter than C tuning.
With either of those, you can have what's called "re-entrant" tuning, where the G (or, for D tuning, the A) string is higher in pitch than the C (or, for D tuning, the D) string. That tuning is (I think) widely considered to be more traditional. However, lots of players tune the G (or A) string to the pitch lower than the C (or D) string. That's the kind of tuning you see in most string instruments (guitars, violins, basses, etc.) where the strings go from lower-pitched to higher-pitched. On a ukulele, it gives the player a bit more range and a wider tonal palette, but you lose the distinctive sound of re-entrant tuning when you play chords.
On a baritone uke, the most common tuning is D G B E, which is exactly like the standard tuning for a guitar's top four strings. It's unusual to find a baritone uke with a high D string.