78 Records or Discs
78s were the most popular form of records from 1900-1946. The term "78 "comes from the number of revolutions or rotational speed the records makes per minute.
Early records were made from rubber and from about 1900 the records were made of shellac. Which is rather brittle. This is best demonstrated from Three Stooges shorts where one usually breaks the record and it shatters. That is how brittle the records were unlike the records of today that can be broken but take some effort to do so.
Shellac remained the substance until 1950. Records then were made out of (basically) unbreakable plastic. This is now commonly referred to as "vinyl." Prior to this these records existed but were never popular because the technology of the day allowed too much surface noise.
1925 saw the introduction of a phonograph run on electricity. This allowed the revolutions to occur at a precise figure. To simplify the motor ran the revolutions at 78.26 rpm (revolutions per minute).
This led to the standardization of that format. However the term 78s didn't come into play till after World War II, when other formats caught on in publicity.
In 1930 Columbia introduced the long play record that played at 33 1/3 revolutions per minute, thus referred to as 33s. The 33 was much less breakable but was much more prone to scratches.
After World War II, Columbia Records kept on with the 33 format while RCA introduced yet another format at 45rpm. In addition to the speeds because of the technological advance during World War II, both formats were far superior in sound to the 78s that used technology from the 1920s.
In order to encourage the consumer to buy their products both Columbia and RCA made their formats incompatible with each other as well as 78s. In addition to the shape of the holes in the record, the needle stylus was also narrower for 33s and 45s and made the 78s play rough. Since Columbia Records and RCA records each had sizable number of recording artists under contract this produced much conflict for the consumer.
These wars continued into the mid 50s when the record industry came to an equalization plan allowing the all the formats to be played on one system.
78s continued to be produced into the 60s in some places like India, and often those records are worth much to collectors.
As a side note there was also a format that played at 16rpm. This speed was useless for music, but well suited for speech. It was used for spoken records as well as some kiddie records.