The PlayStation Controller is the first gamepad released by Sony Computer Entertainment for its PlayStation video game console. The original version (model SCPH-1010) was released alongside the PlayStation on December 3, 1994.
Based on the basic button configuration established with Nintendo's Super NES Controller, the PlayStation Controller added a second pair of shoulder buttons for the middle fingers. Intended to update the gamepad for navigating 3D environments such as the ones PlayStation was designed to generate, the concept behind featuring shoulder buttons for both the index and middle fingers was to implement two-way directional depth controls using the two sets of buttons. To compensate for the less stable grip from shifting the middle fingers' placement to the shoulders, grip handles were added to the controller.
Using the simple geometric shapes Triangle, Circle, Cross, and Square to label its action buttons rather than traditionally used letters or numbers, the PlayStation Controller established a trademark which would be incorporated heavily into the PlayStation brand. In an interview with Teiyu Goto, designer of the original Playstation Controller, he explains what the symbols mean. The circle and x represent yes and no respectively, the triangle symbolizes a point of view and the square is equated to a sheet of paper there to be used to access menus.
On April 2, 1996, Sony released a revised version of the PlayStation Controller (model SCPH-1080), featuring a longer cord with a ferrite bead. After briefly selling the Dual Analog Controller in 1997, Sony began phasing out the PlayStation Controller later that year with the introduction of the DualShock controller, which would become the new standard controller for the PlayStation.
Source: Wikipedia, "PlayStation Controller", available under the CC-BY-SA License.