The number of strings is different: the ukulele has four strings and the guitar has six
Strings are made of different materials: a ukulele is a nylon string, a folk guitar is a steel string, and a classical guitar is a nylon string
Sizes vary: ukuleles are usually around 22 inches and guitars are usually around 40 inches.
The difficulty varies: the ukulele is easier to play than the guitar.
The range is different: the ukulele is in the high register, and the guitar covers almost every range.
The wind is different: the ukulele wind is mostly lively and cheerful, while the guitar can be represented in any style.