There are 4 standard sizes that ukuleles come in. At UkeSchool.com, we concern ourselves only with the 'classic', the smallest, the soprano ukulele. These are what people think of when they think of a uke. They are around 21" long, cute, portable, and fun. The next largest size is the concert uke. At 23", they are tuned the same as a soprano uke, they are just a bit bigger, with a bigger sound. If you want a ukulele that sounds large, that's your thing. All the moves from UkeSchool.com will work just fine.
There are also tenor and baritone ukuleles, which, at 26" and 30", are more like small 4-stringed guitars - they are even tuned differently. They sound a bit like ukuleles and a bit like classical, nylon-stringed guitars. Their sound is more "warm" and less "jumping flea". Most music shops don't sell them, there are almost no books for them, and most online interactive schools of ukulele won't teach you how to play one. Consider them exotic, like a lute, a pedal steel, or bagpipes.
A banjo uke is a type of ukulele that has a body with a tight skin pulled over it, like a banjo. They are very, very, loud. Ukulele guru George Formby used one of these a lot.
A ukulele that has a box-like shape, without the curvy, double-lobed guitar shape, is referred to as a pineapple ukulele. They are vaguely pineapple shaped, but not made from a pineapple.
An armadillo uke, however, is made from an armadillo. The instrument shown to the right is actually a 10-stringed South American instrument called a charango. An armadillo uke is a much more obscure thing, presumably a an engineering feat inspired by charangos. I was only able to find one really crummy picture of a truly ukulelified armadillo, below. Wow. I didn't know these existed until I saw one in for repairs at my local guitar shop. I laughed so hard they almost had to throw me out of the store - turns out that a good specimen of one of these is quite valuable, and sounds very good!
Needless to say, after describing the ones that are made from armadillos, there are many, many types of ukuleles.