A device for the capture of images.
While in modern days we usually think of a camera only in relation to photography (static or motion picture), the concept of the camera predates photography by centuries. For example, Leonardo da Vinci used a camera obscura to trace images on paper.
The word camera itself is Latin for chamber. That is because a "chamber" (or, more exactly, some lightproof enclosure) is one of the four parts every camera must have to be considered a camera:
A light-tight box - that is, an enclosure that protects the image from extra light which would destroy it.
This can be a sturdy box made of wood, metal, plastic (bakelite was very popular before the modern plastics evolved), paper, or just about anything. Most cameras made today have a sturdy box.
However, it can also be a flex box, usually a bellows, which offers the photographer a level of control impossible with the sturdy body. This type of body is usually seen only on view cameras these days, though there are a few flex bodies on other types of camera (e.g., Hasselblad makes a medium format flex body camera).
A pinhole - that is, an opening for the image to enter. This is typically very small, a pinhole with the diameter measured in microns is not unusual.
A typical modern camera enlarges the pinhole and places a lens, or an objective (a lens assembly) in front of the pinhole. This allows more light to enter but at the cost of some sharpness of the image (when a pure pinhole is used everything is equally sharp). The loss of sharpness is not necessarily a disadvantage: Many a picture has been improved greatly by leaving the background blurred.
A shutter - that is, some means of turning the light on and off. In other words, a way to open and close the pinhole.
In early days of photography, when film was not very sensitive, the photographer simply used his hat to block and unblock the pinhole.
Nowadays, the shutter can be quite a complex device, often one of the most expensive parts of a camera (though a good lens will costs even more).
In digital cameras the shutter may not be a separate device as it is in other cameras, but there still needs to be some "time control." In that case, the shutter is simply a function of the software that controls the camera. But it is still there.
A means of capturing the image - for Leonardo da Vinci it was his own hand tracing the image on paper with a pencil, pen, or brush.
For the 19th century photographer, it often was a photo-sensitive glass plate.
For us it can be a film, or an electronic circuit (such as used by TV and video cameras), and lately a digital image processor.
These four parts are all that is necessary to make your own camera, or for any manufacturer to make his or hers. Most recent models contain additional bells and whistles, such as auto-focus, built-in light meters, etc. These new developments make a camera look more like a tiny computer. But they all still have, and have to have, the four parts described above.