Once disks became cheap, it was relegated to use for backups, but it remained popular in that use until recently. It has always had a very low cost per bit, since a reel of tape has much more surface area (and thus space for data) than any other physical form.
It consists of a magnetic medium applied to flexible medium, usually plastic tape (although the UNIVAC I used thin metal tape), and wound on a reel. Mounted on a tape drive, the tape is moved across a head onto a takeup reel, and data can be written to, and read from, the tape as it moves over the head.
For many years the standard width was 1/2"; lengths were typically 1,200 feet and 2,400 feet, on a maxium reel size of 10-1/2". Originally data was stored in 7 parallel tracks (6 data, and 1 parity), but this was soon replaced with 9 tracks (8 data, and 1 parity). 7-track recording densities started at 200 bit per inch, increased to 556, and ended at 800. 9-track started at 800, and increaed to 1600 and 6250.
More recently, open reels of tape started to be supplanted by various types of cartridges; over times, cartridges took over, and open reel tape is now more or less extinct.