Vinyl records come in three varieties, measured in revolutions per minute (rpm): the 33 1/3 rpm, the 78 rpm, and the 45 rpm. Each variety is a different size and revolves at different speeds. The most common records today are the 33 1/3 rpm and the 45 rpm.
A 78 rpm record is 10 inches in diameter. They are rare today, as they were the first format and fit a small amount of music. They eventually became outdated, replaced by 33s and 45s.
33 1/3 rpm
A 33 1/3 rpm record, often called a 33, is 12 inches in diameter. These platters are also called LPs for Long Play, because they fit more music than the 78 rpm records, and they became the standard vinyl for full-length albums.
A 45 rpm record is 7 inches in diameter. 45s are also called SPs for Single Play and are still used widely today for singles.