A song on guitar that has a non-standard tuning.

Standard tuning on a guitar is EADGBE. A song which does not follow this tuning is generally said to have an altered tuning. Some altered tunings include:

"The Rain Song" by Led Zeppelin -- DACGCD
"She Talks to Angels" by The Black Crowes -- EBEG#BE
"Fearless" by Pink Floyd

It is important to note that generally a song which is in a drop d tuning or is tuned down 1/2 step is generally not considered an altered tuning. Partialy because these songs do not change the chord structure significantly.