Tuning a ukulele involves turning the tuning pegs so that the notes emitted by strumming the instrument match the notes supplied by a tuning instrument. Generally the four notes are GCEA from the fourth string (the one closest to the left on the fret) to the first string, respectively.
While the traditional ukulele is tuned to GCEA, alternative tunings are used when tuning specific types of instruments such as soprano, tenor and baritone ukuleles. Electronic tuners display the note one is playing, allowing the player to adjust the tuning pegs to the correct note.
Tuning by piano involves matching the strummed note of a particular string to the corresponding note on the piano. Players can use either a low G (below middle C) or a high G (above middle C) to tune the fourth string.
Relative or standard tuning is a less reliable method whereby a player uses the A string (the one closest to the right side of the fret) to tune the other strings. While the tones may be harmonious with each another, they might not be a true representation of the note they are meant to represent. This method isn't recommended if the player is playing with other musicians.
Some ukulele-centered websites offer online video or audio tuners that play a track of the notes.