Curtain

A curtain is a piece of cloth intended to block or obscure light, or drafts, or (in the case of a shower curtain) water. A curtain is also the movable screen or drape in a theater that separates the stage from the auditorium or that serves as a backdrop/background.
Curtains are often hung on the inside of a building's windows to block the passage of light. For instance, at night to aid sleeping, or to stop light from escaping outside the building (stopping people outside from being able to see inside, often for privacy reasons). In this application, they are also known as "draperies". Curtains hung over a doorway are known as portières. Curtains come in a variety of shapes, materials, sizes, colors, and patterns. They often have their own sections within department stores, while some shops are completely dedicated to selling curtains.
Curtains vary according to cleanability, ultraviolet light deterioration, oil and dust retention, noise absorption, fire resistance, and life span. Curtains may be operated by hand, with cords, by press-button pads or remote-controlled computers. They are held out of the way of the window by means of curtain tie-backs. Measuring curtain sizes needed for each window varies greatly according to the type of curtain needed, window size, and type and weight of curtain.
Curtains are a form of window treatment and complete the overall appearance of the interior of the house. Window treatment helps control the ambiance and flow of natural light into the room. The effect of drapery or curtains is best seen in daylight, and with proper indoor light positioning, can look attractive even at night.
History.
From evidence found in excavation sites at Olynthus, Pompeii and Herculaneum, portieres, a curtain hung over a doorway, appear to have been used as room dividers in classic antiquity. Mosaics from the 2nd to 6th century show curtains suspended from rods spanning arches.
England.
In England, curtains began to replace wooden shutters towards the end of the 16th century. In Medieval England, the earliest form of window treatments were leather panels threaded onto iron rods. These were eventually replaced with woven wool panels. During the reign of Elizabeth I, ornately decorated Italian Renaissance fabrics, including brocades, velvets, and damask, began to be imported. These ornate fabrics, as well as fabrics decorated with crewel embroidery, were used in curtains during the English Elizabethan and Jacobean periods. Solid wood shutters were used during cold seasons.
Light control and insulation.
Curtains are manufactured from a variety of thick fabrics, each with a differing degree of light absorption and heat insulating qualities. For maximum temperature control, the curtain gap to the window should be small, with minimum convection drafts below or above the curtain. Various architectural structures around the curtain can minimize these air drafts, but usually they are just used for decoration and make rooms feel more cozy.
A sheer or net curtain is one that is made from translucent fabric, such as a loosely woven polyester voile, silk or nylon made marquisette or ninon, and cotton lace, etc. Sheer curtains allow a majority of light to be transmitted through the fabric, with the fabric weave providing a basic level of UV protection while retaining maximum visibility outward through the curtain. Sheer curtains are sometimes referred to as "privacy curtains" in reference to their screening abilities; during the day most sheer fabrics will allow people inside the home to see the outside view while preventing people outside the home from seeing directly into the home. Due to the loose weave in sheer fabrics, these types of curtains offer very little in the way of heat insulation.
Uncoated fabrics provide the next level of heat insulation and light absorption. Uncoated fabrics constitute the vast majority of fabrics used in curtains, and are composed of a tightly woven fabric, most typically a cotton/polyester blend, which is mostly opaque when viewed in ambient light. Uncoated fabrics provide a reasonable level of heat insulation due to the tight weave of the fabric. However, the fabric itself is typically not thick enough to completely absorb strong light sources. As a result, when curtains made from uncoated fabrics are closed in an attempt to block out direct sunlight, light will still be visible through the curtain.
Coated fabrics consist of a standard uncoated fabric with an opaque rubber backing applied to the rear of the fabric to provide improved light absorption. To create a coated fabric, a liquefied rubber polymer is applied in a single coat to an uncoated fabric and subsequently fused dry by means of a heated roller, in much the same way that a laser printer applies toner to a sheet of paper before fusing it dry. A fabric that has been through the coating process once is considered a "1-pass-coated" fabric, anecdotally referred to as "dim-out" or "blackout" because of the fabric's ability to absorb approximately 50-70% of a direct light source. To improve the light absorption of a fabric it is possible to re-coat a fabric up to a maximum of "3-pass-coated", which is considered sufficient to block out 100% of a direct light source, hence such fabrics are referred to as "blockout-coated".
Maximum light absorption and heat insulation in a curtain is created through a lined curtain, which typically consists of an uncoated fabric at the front to provide the look and feel of the curtain, with a separate coated fabric attached at the rear to provide the insulative qualities. The coated fabric is typically referred to as a lining, which simply refers to a coated fabric that does not have any particular color or pattern.
Curtains may be held back with tie-backs (a loop of cloth, cord, etc., placed around a curtain to hold it open to one side; typically passed through a ring on a hook attached to the wall, and fastened with a knot, button, or velcro; often adorned with tassels) or may be closed and opened with sticks called draw-pulls (rods made of plastic, wood, or metal that can be twisted and/or pulled) or curtain rods which are attached either to the runner or to the first hook. A curtain hook stopper is a device used to stop the curtain from falling off the end of the curtain rail. 
Although some curtains may employ pulley systems, they should not be confused with roller blinds or roller shades. However, they may be generally referred to as window treatments, which are cover or modification of the window, often with the aim to enhance the aesthetics of the window and the room.
Styles.
Curtains can be used to give a room a focal point. There are at least twenty different styles of curtains and draperies which can be used in window treatment.