The size most people play and what most people picture when they think of the ukulele. The usual tuning is A, E, C, G (High to low) with the G on the bottom being only a tone below the high A (Called a re-entry string)
A bigger version of the ukulele, the concert can be tuned the same as the Soprano, but the larger body allows for a more mellow sound with more bass tones.
This is larger again but can still have the same tuning as the soprano. Because of the larger body, it is possible to have the 4th string tuned an octave lower than its smaller companions. This gives a beautiful lower range than is great for playing melody or accompanying.
This is a lot larger and tuned like the first 4 strings of a guitar (E, B, G, D) This makes the baritone a very popular choice for guitarists who want a different sound.