A laser is merely a light source which has the property that the light emitted is focussed as a narrow beam with very little divergence. This is unlike a regular light bulb that emits light in many directions in a variety of frequencies. Due to the property of low divergence of the beam, lasers can be used to indicate the aimed point on a target.
The original laser devices were extremely large and bulky, but due to advancement of semiconductor technology, they can be made much smaller these days. The typical laser sight today is small enough to be mounted to the underside or top of a pistol.
The laser sight is attached so that it is parallel to the barrel. Since a laser beam does not diverge much, the user can move the barrel until the light spot from the laser hits the desired target. The spot indicates the area where the barrel is pointing to. It must be remembered though, that the light travels in a straight line, but real bullets travel in an arc, due to forces of gravity, wind etc. The user may therefore need to allow for windage and elevation depending on the distance between the weapon and the target.
Most laser sights utilize red laser diodes. In the late 1990s, green laser diodes were invented, but it wasn't until 2007 that the first mass produced green laser sight was invented. Green laser sights are a bit more expensive than red laser sights and consume more battery power. However, green laser light is much more visible to human eyes than red laser light, especially in bright daylight conditions.
The two pictures above show a red laser and a green laser during day time and night time. In the bright day time shot, the red laser dot is barely visible in the picture, whereas the green dot is easily seen. The green dot is also seen much more clearly at night time, where the entire beam is clearly visible, not just the dot. With red lasers, only the dot is visible normally and the beam is usually only discernible when the surroundings have smoke or dust. This makes weapons using green lasers much more easier to aim than red laser sights.
One of the disadvantages of red and green laser systems is that the target (and other people surrounding the target) can also see the dot and be aware that he or she is being targeted. Also, the visible beam gives away the position of the person pointing the weapon. Therefore, some systems use an infrared laser that is only visible to people wearing night vision devices. Unless the target is also wearing a night vision device, he or she is not aware of being targeted and doesn't know where the person aiming the weapon is either.
Laser sights allow the user to quickly acquire a target. With other types of sights, the user needs to concentrate on pointing the sights to the target and therefore loses focus of things that are surrounding the target. Laser sights allow the user to not lose details of the target's surroundings. It also allows one to aim the weapon without physically staring down the barrel, which allows for greater concealment.
On the other hand, laser sights need batteries and there is the chance of the batteries running out when they are needed. Red laser beams are hard to see in daylight or in clean surroundings. Green lasers are more visible in bright or clear conditions, but they chew through batteries much more quickly than red lasers and are also more expensive than red lasers. Laser sights also alter the balance of the weapon somewhat and rough usage can change the spot where the laser is pointing, which means it no longer points to where the barrel is pointing.