Iron Sights are a network of metal notches or points to facilitate the aiming of firearms, bows, crossbows, and other objects used to fire projectiles. The rear sight is usually either an aperture sight (closed sight) or a notch (open sight). The front sight is either a bead, ring, or post. The front sight may or may not be hooded to protect the front sight post.
When using open iron sights, the tip of the front sight post is centered between the two rear sight notches, with the height of the tip not exceeding the height of the notches. The point at which the sights line up is the point of aim.
When using closed iron sights, the tip of the front sight post is centered in the rear sight aperture.
In some weapons, the rear sight is adjusted for windage (left/right adjustment) and range (up/down adjustment - not all weapons have this functionality, however); the front sight is usually adjustable for elevation (up/down adjustment).
In other weapons, the sight system is a bit different. The rear sight is used only for range, and the front sight is used for windage and elevation.
On some rifles equipped with optics, there are iron sights attached that may be used in the event of a scope malfunction, or if the level of magnification that the scope provides is considered unnecessary (in close quarters, for instance). These are known as BUIS (back up iron sights).