So I take it you've never seen one...
The 8-track cartridge uses 1/4-inch wide magnetic tape in a large loop, with a single reel inside. It's a stereo system, and to hear the whole tape you run the loop through four times. The track width comes out about the same as for a cassette tape, but the 8-track runs at 3-3/4 i,p.s. (double the cassette's speed), so the frequency response and S/N ratio tended to be better.
The deck contains the motor-driven capstan, but the associated rubber pinch roller is fitted inside each cartridge.
Here's an 8-track tape with a compact cassette alongside for comparison:
If you look at the "business end," you can see the pinch roller on the left:
The "end" of the tape is marked by a short metal foil section. When this bridges two contacts it causes a solenoid to physically step the tape head into position for the next program. The four programs combined typically give about the same playing time as an LP record album.
You can fast forward the tape to advance or get back to the beginning of the loop, but there's no rewind facility. Players generally have a manual program select button so that you can start on any one of the four programs you want, or jump programs at will. If you leave it on continuous play, the cartridge just keeps repeating over and over, which made the system quite popular for background music in restaurants, etc.
[This message has been edited by pauluk (edited 05-22-2003).]