Photography is built on the three pillars of exposure: (1) shutter speed, (2) aperture and (3) sensitivity.

Exposure is the moment when light strikes the sensor and the image is recorded.

Three factors combine to determine the correct exposure for a digital image:

a) The amount of light in the scene strikes the CCD or CMOS sensor
b) The length of time that sensor is exposed to the light
c) The sensitivity of the sensor

An aperture is the variable opening in a lens that allows light through.It is made up of a series of overlapping metal blades called an Iris diaphragm to allow the size of the aperture to vary from full diameter to small hole.

This variable opening determines the amount of light that falls on the sensor. The smaller the f-number (e.g. f-2.8), the bigger the aperture size.

Big apertures like f2.8 or f4 have a shadow depth of field, which will produce a picture with foreground sharp, but the background soft/blur.

Smaller apertures like f22 have a greater depth of field, which will produce a picture with both foreground and background sharp.

Shutter is the device that opens and closes to control the time that light is allowed to pass through the aperture to the sensor.

It is measured in seconds and can range from 1/4000 for very fast camera to 15 seconds. Shutter speed control how moving image will appear in the picture.

Fast shutter speed will freeze moving action, while a slower shutter speed will create a "motion blur" effect.

What is ISO?
It is the term used in the traditional film photography to indicate the sensitivity to light of the film roll. The higher the number, the more sensitive it is.

ISO 400 is twice as sensitive to light as ISO 200, and thus can perform better in lower lighting condition than ISO 200.

In DLSR, of course there is no more film used. Instead, the ISO setting will tell the sensor how to behave if it is a traditional film.