The ukulele is a 19th century Hawaiian instrument with a basic construction very similar to the guitar. The ukulele is generally built from wood in a shape that mimics an acoustic guitar. Although the soprano ukulele is by far the most common size, there are four sizes of ukuleles - the soprano, concert (alto), tenor and baritone.
The basic technique for strumming a ukulele involves striking the strings with the nail on the index finger on the downstrokes, and the pad of the index finger on the upstrokes. Some ukulele players instead choose to use their thumb to strum. The use of a felt pick in strumming the ukulele is also sometimes utilized.
Typical tuning for a four-string soprano mandolin differs from guitar, in that the strings do not progress from low to high. Instead, on the ukulele the lowest string (the string closest to your face) is a G - the second highest note of the four strings. Following this, the third string is C, the second string is E, and the first string is A. For clearer insight into tuning on the ukulele, read this tutorial on how to tune your ukulele.
Concert (alto) ukuleles are more often tuned to GCEA. Tenor ukuleles are also most commonly tuned to GCEA. And baritone ukuleles are most often tuned to DGBE.
If you've just gotten your hands on a uke, and are looking to get started, take a look at 9 ukulele chords you should know. For some great ukulele insight, including some very well done beginner lessons and buying advice, I'd recommend visiting the wonderful Ukulelehunt.com.