The first difference has to be the number of strings available. Although the sound of the ukulele is very much similar to the guitar, there would still be slight differences with the tune of the ukulele when compared to the guitar. Another difference is the higher tuning but this is something that can easily be used.
Some say that the ukulele is an easier version of the guitar but it should be remembered that some of the frets are small and are so much harder to play as compared to the guitar. It is not always “easy” to play the ukulele especially when there are some songs that you want to play that are truly complicated.