A picture frame is a container added to a picture in order to enhance it, make it easier to display or, in some cases, to protect it.
The picture frame may contain a pane of glass or a plastic glass substitute such as Plexiglas, in order to protect the picture. In some instances where the art in the frame is dispensable or durable, no protection may be necessary. Glass is common over watercolours, rare over oil paintings. Glass may be treated with various coatings, the most common being UV filters. Some glazings such as Denglas, Optium, and Museum glass have a coating which makes the glass partially non reflective and practically invisible under ideal lighting conditions.
For pieces to be framed under glass, except for the most disposable and inexpensive posters or temporary displays, the glass must be raised off the surface of the paper. This is done by means of matting, a lining of plastic "spacers", shadowboxing, stacking two mouldings with the glass in between, and various other methods. If the paper (or other media) were to touch the glass directly, any condensation inside the glass would absorb directly into the art, having no room to evaporate. This is harmful to almost any medium. It causes art sticking to the glass, mildew, and other ill effects. Raising the glass is also necessary when a piece is done in a loose media such as charcoal or pastel, to prevent smudging. Care should be taken with these works, however, if Plexiglas is used as a static charge can build up which will attract the pigment particles off the paper. Using real glass helps to prevent this.
Certain kinds of pieces do not usually need glass when framed, including paintings done in acrylic or oil paint (the former is usually waterproof; the latter actually needs to "breathe" due to the decades-long drying process), stained glass or tiles, and laminated posters. These kinds of pieces are still sometimes put under glass though if for example, they are framed using mats, or (in the case of oil paintings) they are kept in a carefully climate-controlled environment.
A photo cube is a special type of picture frame, often used to show several different members of a family, parts of a vacation, or a timeline of someone's life.
Other styles are clip frames, box frames and shadow boxes, among many others. A digital photo frame is an example of the changing technology of the 21st century.