It is an instrumental and was named after the star constellation, Orion, due to the song's spacey sounding bridge. It was written by James Hetfield, Lars Ulrich, and most notably, Cliff Burton. The song is more progressive than the band's previous work.
There are three main parts of the song: the heavy beginning, the spacey bass-led interlude, and the ending recapitulation that brings back the heaviness. The song features four solos: the first one by Kirk Hammett, the bridge by Cliff Burton, the third, a bass solo by Cliff Burton shortly after his bass interlude, and the last by Hammett again.
In an episode of "It's Electric," Lars and Kirk reveal that during the writing stage, Orion had actually been part of the song Welcome Home (Sanitarium), under the name "Only Thing".
This song was played at the memorial for Cliff Burton, most notably because of the bass solo. It was also played live in 2006 when Metallica played the entire Master of Puppets album. In honor of Cliff, the band named it's festival "Orion music and more".
Metallica played Orion during the Escape from the Studio '06 tour, as well as Rock in Rio 2011. During their '05 gigs, they played Orion as their first song. During their Damaged Justice tour, Kirk would play his portions of the song. During the Wherever We May Roam Tour after Jason Newsted's bass solo they play a jam of the song. A live performance can be found on the Live Shit: Binge & Purge. During the Nowhere Else To Roam Tour they would combine "Orion", "To Live Is to Die", and "The Call of Ktulu" and form them into a medley. On November 13, 2005, "Orion" was played for the first time ever in San Francisco, CA but it was shortened it wouldn't be played in its entirety live until June 3, 2006, in Nürburg, Germany. As of February 20, 2018, "Orion" has been played live in its entirety 68 times.