Phaser
Phasers are the primary energy weapon used in Star Trek. They can scale from concealable hand weapons to starship weapons.
Nature
Phasers are particle beams that usually heat materials they strike. They often start chain reactions of an unknown nature that continue well after the beam stops striking the target. Phasers at full power have been known to make their targets completely disappear, an effect often described as "vaporization", although it is never accompanied by the violent explosion and superheated cloud of vapor one would expect from vaporizing a target such as a human in about a second.
The operating principle of phasers -- whatever it may be -- makes them more effective against some materials than others. While they can make human tissue disappear in a moment, they are completely ineffective against some materials, such as "toranium inlay"[1].
Power Settings
From their first appearance in continuity[2], hand phasers have had variable power settings. A "stun" setting can incapacitate targets for capture without causing serious harm; one incident[3] has shown that even starship phasers have this capability, enabling them to stun large numbers of people from orbit.
Phasers also have a "kill" setting which ranges in effect from causing serious injury to making targets disappear entirely[4]. Phasers set to kill have also been used to cut through tree trunks[5], heat rocks to serve as make-shift campfires[6], and blast through rock[7][8].
Trekkies have claimed that a single shot from a hand-phaser can rearrange the landscape by bringing down buildings or mountains. Despite one supporting claim from a deranged character[9], hand-phasers have never demonstrated firepower of this magnitude, and characters routinely protect themselves from phaser fire by hiding behind ordinary rock formations[10] or even packing crates[11]. Phasers certainly can blast through stone, but Star Trek combatants almost never use such settings in combat, indicating that doing so would have disadvantages, such as rapid depletion of the weapon's power supply.
As noted, phasers on high power settings can make living targets disappear. Trekkies have argued that such targets are "vaporized" and used such incidents to calculate phaser firepower based on the energy required to boil 50-100 kg of water into steam, but phasers "vaporization" incidents never produce the large cloud of super-heated vapor that would inevitably result from actually vaporizing a human being. Consequently, some other effect must be occurring in such incidents, making it impossible to calculate hand-phaser firepower from "vaporization".
Beam Adjustment
Phasers typically produce a narrow beam, but they can be adjusted to produce a cone- or fan- shaped "spray" of energy. These "wide-beam" shots have been shown to be capable of simultaneously stunning multiple people in the affected area at a range of several meters[12].
Despite the potential to affect many targets and suppress areas of a battlefield with wide-beam phaser fire, Federation troops invariably use only narrow beams in battle. This behavior indicates that wide beams have extremely limited utility in combat situations. One obvious reason is that the power intensity of spreading beams will drop rapidly with increasing range, making them ineffective at typical combat ranges.