What is a Ukulele ?
The ukulele is a small guitar-like instrument brought to Hawaii by Portuguese immigrants at the end of the XIX century. It was popularized during the Panama Pacific International Exposition in 1915, and later became an icon of the Jazz Age in the twenties. From the late 1940s to the late 1960s, plastics manufacturer Mario Maccaferri turned out about 9 million inexpensive ukuleles, but the ukulele then declined in popularity until the late 1990s. Highly portable and relatively inexpensive, the ukulele is nowadays very fashionable. Some of the most famous ukulele players are Ernest Kaai, Benny Nawahi , Israel Kamakawiwo'ole (known for his 'Somewhere Over The Rainbow' cover), and George Formby. There are four common sizes of ukuleles : soprano, concert, tenor, and baritone.
A tablature is a method of writing down music played on stringed instruments using ASCII characters and numbers. It's a visual representation of the instrument's fretboard.
The 4 lines correspond to the strings of the instrument,
A ukulele tablature is made of 4 lines representing the strings of the instrument, with numbers showing where to fret the strings. Extra symbols shows how to play the notes.
Tablature is more or less the general name given to online sheet music, often just made of chords & lyrics.
More more infomation : How to read a ukulele tablature