There are few instruments in the world that are likely to captivate your imagination the way the ukulele will. I began playing in 2006 and have never looked back. So please take a moment to properly prepare your psyche for the addiction your about to expose yourself to.
First and foremost lets talk about the four different basic types of ukuleles.
The Soprano Ukulele is the highest pitched and smallest of the Uke family. This is the most commonly pictured size in pictures and pop culture for its folk heritage and link to the Hawaiian Islands. The standard soprano has a 13" scale with traditionally 15 frets although different makers have been known to make some with a few more but I have yet to see any with less than 15. As with all ukuleles, different models and brands come made in a variety of materials although any respectable model will be made with wood. Different types of woods produce different tonalities and greatly influence the 'voicing' and volume of the instruments. If your coming from another stringed instrument you may want to consider a different size Uke as the exceptionally small fretboard can make it difficult to adjust your fingering correctly. If your a newcomer, the Soprano is a great choice and tends to be the most affordable until you get into the professional level Ukes.
The Concert Ukulele is a less popular size but should not be overlooked. The concert adds an extra 2" inches to the length of the Sopranos fretboard and that makes a huge difference to someone looking for the Soprano sound with extra room on the long side. This is an excellent choice for crossovers from other instruments and for newcomers alike. Due to the added mass of their body, they are louder and have better sustain than Sopranos although due to being less desired, quality sub $100 ones are hard to come by.
Tenor is the big brother of the Uke world and deserves its fair share of respect. You will find that a number of the modern "greats" prefer the Tenor for its rich, balanced sound that blends together aspects of Uke and Guitar and lends itself better as a 'professional' style instrument in some circles. Clocking in at 17" the Tenor gives a comfortable amount of neck room to position and chords and also adds additional frets for soloing and playing practically any type of music you can imagine. Quality Tenors run in the $150+ and fortunately dont seem to be popular among brands producing toy quality ukes.
Last but not least we come to the big, deep baritone! The jilted uncle of the uke world. All kidding aside, the Baritone will likely be the rarest uke size you come across in your travels as it is somewhat obscure and rather unpopular outside of group situations that call for its heavy bottom end sound. Thats not to say that the Baritone is not fun to play solo, because it is one of my personal favorites, but solo baritone play feels a bit to close to playing a guitar and for me at least it looses some of the 'Uke Magic'. Tuning on the Baritone takes on a dramatic change from its brethren as it is tuned E-A-D-G like a bass guitar which also means all your chords will be played like a hacked down guitar. Great for transitioning players and as mentioned above, Baritone is a must for groups looking to round out their sound.
~Work in Progress~