(covers information from several alternate timelines)
The Library Computer Access and Retrieval System (LCARS or simply library computer for short) was the name of the computer operating system employed by the United Federation of Planets, used aboard Starfleet vessels, starbases and space stations. It was accessible by both voice and keypad commands.
LCARS used sophisticated artificial intelligence routines to understand and execute vocal natural language commands. Starfleet personnel also frequently interfaced with LCARS with PADDs, tricorders and control panels.
In the 24th century, graphical controls housed underneath touch-sensitive clear panels allowed LCARS panels to be quickly reconfigured by users to suit the task at hand, including a tactile interface for visually-impaired officers. (VOY: "Year of Hell") This enabled even complicated tasks to be executed with just a few button presses. LCARS also controlled the retrieval and storage of files in the data banks housed within the ship's computer cores.
Depending on the starship, the LCARS design, especially the color scheme, differed. These colors could range from a yellow/white (found on the USS Enterprise-D) to blue/white scheme (found on the USS Enterprise-E).
Library computer information
- USS Defiant (NCC-1764) library computer
- USS Defiant (NX-74205) library computer
- USS Enterprise library computer
- USS Enterprise-D library computer
- USS Voyager library computer
LCARS are seen in almost every episode of The Next Generation, Deep Space Nine, and Voyager. The interfaces seen in The Original Series and Enterprise are never named and their designs are quite different.
According to a Next Generation episode text commentary by Michael and Denise Okuda, the LCARS displays were rarely actual computer simulations; one such example was the computer screen used by Romulan Commmander Sela to monitor the Federation fleet during her attempt to smuggle weapons and supplies to Lursa and B'Etor during the Klingon civil war in the episode "Redemption II". This was due to the fact, according to the Okudas, of both the high cost and primitive state of computer graphics in the late 1980s and early 1990s. Therefore, most LCARS displays were, in fact, plastic panels with spinning light devices behind them to give the impression that the information on the "displays" was changing.
The full name "Library Computer Access and Retrieval System" can be seen in several episodes, including "The Price".
The sickbay used in Star Trek V: The Final Frontier is a minor redress of the Star Trek: The Next Generation sickbay, and the LCARS panels on the wall are barely altered, so this could be the first LCARS appearance. However, they do not appear in Star Trek VI: The Undiscovered Country, or the 23rd century portion of Star Trek Generations.
In the first two seasons of The Next Generation, large black rectangles are clearly visible on the LCARS displays on the bridge (and sometimes in main engineering). This was a result of the studio lights reflecting off the displays, which director of photography Edward R. Brown tried to solve by sticking cardboard onto them. When Brown was replaced by Marvin V. Rush for the third season, a number of changes in filming (including better film stock and a smaller number of lights) allowed the LCARS displays to be seen properly. 
The LCARS layout has been used in several computer games and applications such as the CD-ROM version of the Star Trek Encyclopedia, as well as the TNG and DS9 CD-ROM Companions. The LCARS layout is also used on the Star Trek PADD app for the Apple iPad.