The word exposure refers to the amount of light falling on the photosensitive material, which is sensor in case of digital camera. A camera is a light proof box with a lens at one end the sensor at the other. The light enters through the lens (aperture) for a certain amount of time (shutter speed) creates an image which is recorded on the sensor. The recorded image is called exposure. In a different lighting condition, the opening of aperture, the shutter opening duration (shutter speed) or the ISO can be different in order to achieve proper exposure.
Proper exposure means the right amount of light recorded on the image sensor. Right amount of light means – In an ideal situation, when we record the image, the dark area of the image should not become black and bright area of image should not become white. In other words, both the dark and bright area of the image should have some details.
Most of the photographers, who migrates from a point and shoot camera to DSLR find it difficult to understand various settings and opts to use auto mode. This way they give full authority to the camera to use its in built program to record the image. The result may be pleasing or not. If it is, they praise the camera, if not, well, for them it becomes a waste of money. DLSR cameras has much more options to control the exposure then a Point and Shoot camera. Therefore it is always advisable to understand the different control points of exposure before we go out for shoot. Understanding these controls gives the photographer the flexibility to be more creative.
When we use manual exposure mode in a DSLR, we are assisted by an exposure meter which helps in accruing correct exposure. It gives us an idea whether our exposure setting for the given lighting condition is correct or not. 
An exposure is controlled by the basic elements which are called Aperture, Shutter, ISO. These three elements form an Exposure Triangle, which will understand in coming sections.
