There are six strings on a guitar, and they are generally tuned in perfect fourths -- EADGBE. This means on a guitar that holding down a string just above the fifth fret on the neck and striking it has the same tone as the open string just below it.
In general, each melodic instrument has a "standard" tuning. That doesn't mean that instruments are always tuned that way, but it does mean that one would expect them to be tuned that way, all things being equal. For the two melodic instruments included in this site, the EADG BE tuning is standard -- those letters represent the tuning of the strings, in order from deepest to highest pitch.
But tuning is not quite that simple. There are quite a few songs that you hear on the radio that are played in variant tunings. Sometimes musicians do this to make their lives easier, sometimes they do this for tonal reasons. For example, KISS often tunes its guitars and bass a whole or half step higher for some of their songs (like "Rock and Roll All Nite"). I always thought they did this to make it easier for Gene Simmons to produce walking bass lines. Perhaps the most well-known musicians who use variant tunings on a regular basis are the Rolling Stones.
Many have been the guitarists who lament that while they can play the right chords for "Wild Horses" or "Bitch," they just can't seem to make the tunes sound quite right. That's because Keith Richards' guitar is often if not usually tuned in what is called "Open G" -- DGDGBD. A guitar tuned this way will produce a G major chord just by strumming the open strings. True to the Stones' blues pedigree, this sort of tuning came out of the Mississippi Delta tradition. It's my speculation, but I believe blues guitarists created Open G out of a need to play chords while they were using a slide on one of their fingers; Open G would allow them to play chords without having to remove it. The simple and ingenious thing about Open G tuning is that to play major chords, a guitarist need only barre a single fret. It also means that to play Keith Richards licks, you can usually walk up and down the neck horizontally using your first and third fingers, once you find a few correct notes. If you have access to a guitar, tune it to Open G sometime and try playing around with it. If you know your Rolling Stones, you can usually figure out the classic tunes without much problem. Jimmy Page also used a variant tuning that was somewhat similar to Open G.
So there are a wide range of tunings that musicians can use to make it easier to play the styles they want. There is an especially good website on Open G and other alternate tunings that can be found here...