In simpler term, we define depth of field as the zone of sharpest focus in front of, behind, and around the subject on which, when lens is focused on a specific subject; with TTL (through the lens) SLR camera, DOF can be previewed in the viewfinder of a camera - the preview is very handy for critical type of work. For an example, when taking a product shot, when you require absolute certain if DOF is adequate to cover the object you intend to photograph Generally, the closer the subject to the camera, the more evenly with the distribution of depth of field in front and behind the subject. As distance of focus extends, DOF usually will be more behind than in front of the focused area.
The "amount" of light allows to strike the film plus the duration (time) for the light to strike the film forms an exposure. The camera has two mechanisms to control exposure, the lens diaphragm (lens section - aperture) and the timing of the OPEN/CLOSE of the shutter curtain (camera section - shutter speed). If this confuses you, the lens diaphragm (inside a typical SLR camera lens) consists of multiple blades which can be open and closed to certain size openings, the variations in the lens opening is called aperture. The size of the aperture determines the amount of light which will fall on the film. Various sizes of the lens opening are indicated by a set /series of numbers called f/stops or f/numbers. Each f/stop represents a specific quantity of light that pass through the lens. The smaller numbers are called large f/stops while the larger numbers are called small f/stops. This is because the larger numbers represent smaller apertures and allow less light to pass through the picture taking lens. Each time you move from one f/stop to the next smaller f/stop (larger number the amount of light allowed through is exactly halved. In effect, the amount of exposure itself is also halved. Using f/2 as an example, the amount of light reaching the film will change according to f/stop as indicated is given in the image below.