Most ukuleles are fairly symmetrical - much more so than an instrument like the guitar. Because of the re-entrant strings - there's very little adjustment needed to convert most ukuleles from a right handed uke to a left handed uke.
However, this is not always the case. Particularly with non-reentrant ukuleles like the baritone and low-G tenor. Because of the differences in string size, it's much more difficult to convert and, therefore, you much more likely to find left handed versions of these ukuleles around.
Also, left handed versions are also made of ukuleles with a cutaway (i.e. part of the body is 'cut away' where the neck joins in order to allow the player to reach higher frets).
Left Handed Beginners
If you're a newcomer to the uke and you're wondering if you need a left handed ukulele, the answer is you probably don't. For most ukuleles, you can just flip the uke, swap the order of the strings and it'll be perfectly fine to play.