"There are two settings: stun and kill. It would be best not to confuse them."
- - Malcolm Reed, 2151
The phase pistol was a type of phase-modulated energy weapon, a personnel sidearm characterized by a focused energy discharge in the form of a steady stream or a phase pulse. It was introduced in 2151 for use by Starfleet personnel as a replacement to the EM-33 pistol. The MACO utilized a different model of phase pistols. (ENT: "Broken Bow", "United")
Starfleet phase pistol
Unlike its predecessor, the EM-33, the phase pistol was equipped with an adjustable setting, allowing the user to choose from either "stun" or, in extreme cases, "kill." While these settings were most commonly used and more readily available, the weapon was capable of an even wider range of output and could be adjusted to a ten megajoule setting (albeit with some internal modification). (ENT: "Regeneration") Also unlike the EM-33, which required the user to compensate for particle drift, the phase pistols were a simple point-and-shoot weapon, making hitting a desired target faster and easier. (ENT: "Sleeping Dogs")
The effects of a phase pistol hit also differed from those experienced with other weapons of that era. 22nd century Suliban and Coridan weapons inflicted identifiable wounds, tearing through clothing and into the victim's flesh. (ENT: "Shadows of P'Jem") The phase pistol, however, produced an injury more comparable to an electric shock, capable of causing severe injury without breaking the skin. (ENT: "Broken Bow")
In addition to use as a weapon, the beam of a phase pistol could be utilized as a cutting tool, capable of slicing through solid rock, a tree trunk, producing heat up to at least 398 degrees. (ENT: "Terra Nova", "Civilization") Phase pistols could also be used as explosives when set to overload; the subsequent explosion causing severe damage in a larger area. (ENT: "United")
Crewmembers from the Enterprise NX-01 first used phase pistols in April of 2151, when Captain Jonathan Archer infiltrated a Suliban helix. Aboard the helix, Archer was able to stun a Suliban soldier before having his own phase pistol turned against him by Silik. Due to the bizarre temporal effects within the helix's temporal communications chamber, however, Archer was able to literally dodge the phase pistol beam and escaped relatively unharmed. (ENT: "Broken Bow") Enterprise carried a total of fifteen phase pistols on the ship during 2151. (ENT: "Shadows of P'Jem")
Ensign Masaro committed suicide with a phase pistol in 2155. (ENT: "Terra Prime") Stafleet phase pistols were used as late as 2161, when an away team from Enterprise fought against a group of aliens on the planet Rigel X. (ENT: "These Are the Voyages...")
MACO phase pistol
MACOs used their own model of phase pistols as their primary sidearm as well. The pistols similar in appearance to the EM-33 pistols and were designed to fire phase pulses instead of beams like their Starfleet counterparts. The pistols were also equipped with both "stun" and "kill" settings.
In 2154, Talas was shot with a MACO phase pistol set to kill. Although she was only grazed, being an Andorian she was especially susceptible to phase pulse infections, and subsequently died of her injuries. (ENT: "Babel One")
Although similar, phase pistols were not yet technically phasers. According to Worf in TNG: "A Matter of Time" phaser weaponry was invented in the 23rd century. Phase pistols were intended as an earlier version of phasers according to the text commentary on "ENT: "Broken Bow"". Even though the Enterprise carried both EM-33 and phase pistol weapons when it was launched, as both were used during "Broken Bow", the EM-33 pistols were not used by the crew again. The phase pistols used by the MACOs in seasons three and four were re-used props of the EM-33 pistols. The MACO weapons were identified as phase pistols in "United". Based on "In a Mirror, Darkly, Part II", the phase pistols were not powerful enough to vaporize a person like a phaser could.
In 2002, Art Asylum produced a phase pistol toy available at most toy stores at the time. The mold was so accurate that, although approximately 20% smaller than the prop created for the series, Enterprise producers ultimately used the toy versions as on-screen props, for stunts and some cases full-on "hero" shots. They were repainted, since when the Art Asylum toys were released they were not the same colors as the ones they used already on the set. While original "hero" props were built for US$2,500, the toy version was priced at just US$20 and came bundled with a communicator. At a convention, actor Scott Bakula had an opportunity to pose with one of the toys and he apparently said that it felt even more realistic than the prop version. (citation needed • edit)
Several prop phase pistols were sold off on the It's A Wrap! sale and auction on eBay.  The MACO pistol was called a hand gun  and a plasma gun  and in some cases still identified as an EM-33 energy gun  in the auction.