A capo will allow you to play in different keys much more easily. It will hold down all the strings at a certain fret - like having an extra finger. For example, if you are in C-tuning, putting a capo at the second fret will mean your uke is in D-tuning.
There are two broad types of capo.
Elasticated capos: These are attached to your uke by elasticated fabric. This type is much cheaper but less pleasant to use. I've fumbled and stumbled with this type of capo on many occasions - but I am clumsy, so you might not have the same problem.
Lever capos: This type of capo is attached by a lever which clamps onto your uke. They are quicker to put on and take off and tend to create fewer buzzing problems. You won't offer see this type of capo advertised as a ukulele capo. But if you buy a mandolin capo, they will fit the ukulele fine.
It is possible to use a guitar capo on the ukulele. However, it is much larger and will restrict your playing particularly on the lower frets.