Sony is a Japanese conglomerate corporation. Its emphasis is on electronics, media, and entertainment.
In 1946, after the second World War, a radio repairman named Masaru Ibuka founded the Tokyo Telecommunications Engineering Corporation in a bombed out department store in Tokyo. Starting with less than $600 in capital, the company created Japan's first tape recorder, before changing the company's name to Sony in 1958. Meanwhile, the company made some of Japan's first transistor radios, ultimately becoming a leader in the field of radio hardware.
Sony only expanded their business from there, creating a life insurance division, other electronic hardware, and music. Along the way, Sony constantly innovated, creating new ways of storing and sharing data, including the video cassette, the Walkman, and, most notably, the CD-ROM. Sony also expanded their business abroad, becoming part of the "Japanese invasion" of America in the 1980's, along with Nintendo.
Incidentally, Nintendo and Sony at one point planned to collaborate on a games console, using Nintendo's expertise and Sony's CD-ROM technology to create a new console called the "PlayStation." However, Nintendo backed out of the deal at the last minute, humiliating their partner. Despite this insult, however, Sony and its president Norio Ohga did not choose to become Nintendo's competitor, instead focusing on their other enterprises.
Since 1990, Sony has expanded into other industries, acquiring large music and film components. These components have become major parts of its business, but largely unprofitable for Sony. Although still one of Japan's largest corporations, Sony has experienced losses these past few years.
Since collaborating on the aborted PlayStation, Sony has had minimal direct involvement in the video game industry. Sony owns no development studios, nor do they have a publishing division. That said, they do frequently have movies with video game tie-ins. These games are frequently published by publishers such as Activision. It is worth noting that many of these movie games appear on Nintendo systems, despite the long-standing animosity between the two companies.