When you refer to pistol grips you are referring to the portions of pistols which are held in the hand. These grips allow the pistol to be gripped so that the pistol is held in a forward position that allows the pistols to be also held properly oriented in the vertical axis also.
A pistol grip is also so designed, so that the hand that grips the pistol is also in a position to operate the trigger at the same time. A pistol grip also serves at times as part of the storage for magazines or other tools. It can also be a part of the mechanism that facilitates the charging of the pistol. When the pistol grip is held in the hand between the palm and the fingers, the forefinger is in a position to pull the trigger, whereas the thumb is clear to release the safety catch, without having to move the palm or other fingers.
Pistol grips are made of wood, rubber and other composite materials. The size of a pistol grip will vary depending on the size of the hand, length of the finger and other comfort factors. When a pistol or handgun is aligned with bones of the forearm then the recoil from the pistol is absorbed. The pistol grips in use must allow such alignment to be achieved easily and without any problem. The hand holding the pistol grip needs to be as close to the bore of the pistol so that the recoil felt is less and also allows recovery between shots. Good shooting practice also requires the supporting hand to be held under the trigger, with the fingers parallel to the shooting hand.
Pistol grips are the only part of any handgun that can be customized and made into very personal attachments. Just changing the design and the color of the hand grip allows you to give any pistol a very personal look that is unique. While looks and color can be of importance, any such customization also has to pay very serious attention to the way it can promote the way you are able to handle the weapon and operate it.
Pistol grips must allow a gun to point at any target naturally. It must also allow the person operating the weapon to waste very little time in aligning the sights. Delays here, especially in law and order situations can make a difference between life and death. The pistol grip should allow the gun to remain in the same position after a shot is fired, without requiring any conscious effort on the part of the user. It should be able to use the straight thumbs method of holding the pistol, which is now considered the best way of holding a gun. In this method, the supporting hand below the trigger guard has the thumb pointing to the front. The thumb of the shooting hand, which normal operates the safety mechanism is also rested on this thumb, also aligned to point forward.