The 45 was a vinyl record. Prior to the rise of vinyl, records had been produced using shellac and synthetic resins. They held up well but could also be broken easily because of their lack of flexibility. There were no 45 records produced until after vinyl became the industry standard. The 45 record was a 7-inch vinyl disc first produced by RCA Victor, a smaller and high-quality record to rival Columbia's 12-inch record.
A 45 was called a "45" for the same reason the larger records were called 33s --- it's the speed at which they turn to make the proper sound. A 45 record spins on the turntable at 45 revolutions per minute to re-create the sound recorded in the studio by the musicians. If it spins faster, it takes on a higher-pitched sound, while slowing it down makes the music drag and the vocals sound like some kind of monster. The 33 record was spun at 33 rpm for the same reasons. The new formats had to have record players with "45" and "33" settings in order to play correctly, but the availability of the new players made the new vinyl formats very popular beginning in the late 1940s.
There is a difference in the 45 and the 33 aside from the speed at which they play. They also had a different niche in the music industry. The 45 record was known as a "single." These records were the ones sold in record stores that contained only one selected song that producers thought would be popular. There was usually another accompanying song on the flip side known as a "B-side track." Hit songs were eventually measured by the sales of singles. These singles were often a catalyst to get music lovers to buy the entire album. The album was typically recorded on a 33-rpm record because it was larger and able to hold many more songs. The 45-rpm single was most prominent as a measuring stick for success during the rise of rock 'n' roll in the 1950s.
While as of publication 45 records are still available in specialty record shops and are even still released by some musical groups, they are nowhere near as common as they once were. The 45 never recovered from a decline that began in the late '60s. People began buying other formats, including 8-track tapes, cassettes and eventually CDs and digital tracks. According to the 45-rpm website, when the Beatles first hit the scene in the early 1960s, 45 record sales would need to top 750,000 to make a No. 1-charting record. But 10 years later, a band could accomplish the same feat by selling 150,000 45 records. This was a true indicator of their decline.