What is Exposure?

The combination of shutter speed, aperture and ISO is obtained as Exposure

i.e. Shutter speed + aperture + ISO = Exposure.

If you are not using these things then probably you wouldn't need an SLR just go for point-n-shoot cameras.

You can easily understand the working and need of exposure setting through this image:

All the three components are provided with numerical values and one can result in ideal exposure according to the condition by adjusting these values.  

All the three components have their distinct function as shown below:

Now, how, where and when we can use these settings to obtain a good image?

These settings are co-related; always we have to maintain the exposure level by adjusting these values. It depends on what we are shooting and what we need to adjust, for example: If we are shooting something like sports or waterfall or anything in motion then we should play with the shutter speed.

Let see:

We want to freeze the moving object which required 500 shutter speed, which result in low light then we do adjust. The image will result in good light at 125 shutter speed but we want to shoot it at 500 which is two steps higher and causing darkness.

We can maintain the settings by adjusting the two steps:

Aperture f5.6 (1-step brighter)	Aperture f4 (2-step brighter)	Aperture f8 (same)
Shutter 500 (2-step darker)	Shutter 500 (2-step darker)	Shutter 500 (2-step darker)
ISO 800 (1-step brighter)	ISO 400 (same)	ISO 1600 (2-step brighter)

Similarly we can adjust other things too.

Have a good shoot.