A hologram is a construction of photons, forceshields, and holomatter created inside a holodeck, a holosuite, or by another type of holographic projector. Holograms could be manipulated by advanced computer programs, allowing people to interact with the inanimate objects. Holograms can also be projected into space. Highly advanced holoprograms, like an Emergency Medical Hologram has the ability to grow beyond its programming and become sentient.
Holodecks are often standard on Starfleet ships but due to their large energy requirement some smaller ships do not carry them on board. Some Federation facilities carry simplistic holograms to perform menial tasks. Sometimes more advanced holograms are used for such purposes too, this remains a point of controversy within the Federation.
By the 24th century, holograms were a standard part of any Federation life, but generally were not aware that they were "not real."
On stardate 48315, Dr. Lewis Zimmerman created the first functional replacement hologram known as the Emergency Medical Hologram or EMH for short. These holograms were meant to be used for a short period of time if a ships medical staff needed help or was incapacitated. Unsatisfied with the program it was developed until there were four versions of this program. The Mark 1s soon became obsolete and were sent to work in dilithium mines.
The first hologram who was able to exercise its true sentience was the EMH of the USS Voyager. His development was slow but he was eventually accepted as the crew's friend. He was even enabled more freedom through use of his mobile emitter, a piece of 29th century technology, which could store a hologram's program.
In 2374 The Doctor was instrumental in creating another sentient hologram called Sek. Based on The Doctor's own matrix and on the personality file of Kes, a former Voayger crewmember that served as a nurse, this program was developed to function as full time nurse in sickbay. Its creation required an enormous amount of energy and its outcome or success was unsure. But due to an urgent medical emergency that required an experienced nurse to aid The Doctor captain Kathryn Janeway approved of its creation.
Lieutenant commander Tuvok however, raised some objections to the creation of Sek. He found the use of Kes' personality and appearance distasteful and against Starfleet regulations. Further more, he also found the idea of creating holograms with the ability to become sentient troubling, mostly due to the fact that one would only create them to serve a certain function, essentially making them slaves. Because of his objections, captain Janeway issued a directive on Voyager that prohibited the creation of any further holograms as advanced as The Doctor. She also gave both The Doctor and Sek full crewmember rights and officially recognized them as sentient beings. In essence captain Janeway struck the first blow in hologram rights. (JB: "Just Between Friends")
In 2379 Starfleet medical attempted to have The Doctor declared non-sentient. They did this to gain access to his program, most likely in order to delete him. This was because many medical practitioners became concerned that holographic personnel like The Doctor would eventually replace them on starships. With the help of his former crew mates and his laywer Samantha T. Cogley The Doctor eventually was given full sentient lifeform rights and recognized as such within the Federation. However, fearing for his existence The Doctor had already resigned his commission and fled the Federation jurisdiction. He later returned to the Federation after being informed of the court ruling and rejoined Starfleet in 2380.