In order to be as accurate as possible, let's go directly to the manufacturer. This information was copied from the back of the package of my vintage, mint-condition,unopened, unrecorded 8-track tape, manufactured who knows when by Radio Shack (R), brand name Realistic(R), factory number #1093073:
This continuous-loop magnetic tape has been developed especially for self-contained automatic operation without rewinding or re-threading. By contiuous-loop, we mean that the tape, in effect, is endless. You may observe that the back side of the tape (which is dark grey in appearance) seems to have a sheen. This is the application of a specially prepared lubricant formulation which has been specifically engineered for these continuous-loop cartridges. This lubricant permits the tape to flow freely, thus increasing tape life as well as decreasing head wear.
Just think.. an 8-track tape is endless!
But back to the subject: how 8-tracks work.
You can see that the tape goes from left to right. The tape is first pulled from the insideof the reel, out, then around the plastic guides,across the head and played, and then is pulled by a turning shaft around a wheel, back down to the outside of the reel. The fact that the tape is physically contacted by the player's mechanism, dragged, stretched, and pulled along instead of reeled just maybe, might possibly be the reason for the short physical lifespan and low fidelity of the recordings.
You can also see that, as the player selects programs, which consist of two tracks, the head moves up and down to accomodate for each program. There are usually four programs to a tape, all four running by simultaneously, but only one being played at once. If you attempt to play an 8-track's reel on a 4-track reel-to-reel tape machine, you'll end up playing two songs at once. This can also happen on an 8-track player if the tape is worn out, and/or slightly misaligned.