The intensity, or brightness, of objects in the real world operate a lot differently than in a photograph. In reality, the brightness of an object, for all intents and purposes, is infinite. A powerful light bulb might look particularly bright, yet compared to the sun, its rather dim. The overall range of possible levels of light intensity is immense in the real world, ranging from dim starlight (say 0.0001 on a hypothetical scale) to sunlight (100,000,000 on the same hypothetical scale). This range of intensity is what we call dynamic range.

The human eye is capable of perceiving a limited dynamic range, and it cannot see both the dimness of starlight and the brilliance of sunlight at the same time...you can see one or the other. If your eyes are adjusted to see starlight, the sun and anything lit by it would effectively be "clipped" as far a your vision and perception are concerned. Conversely, if your eyes are adjusted to see the world illuminated by sunlight, the dimness of starlight would be well below the darkest parts of the world around you...effectively clipped into shadow. What is amazing about the eye, however, is its ability to adapt...the total dynamic range the eye is capable of functioning is extremely large...smaller than the total range of possible intensities, but far larger than common electronic devices like cameras and computer monitors.

Similarly, camera sensors and computer screens have an even more limited dynamic range than the human eye. Different than the eye, however, is the fact that digital devices must represent dynamic range as discrete values capable of being represented digitally. Digital devices are also limited in the total range they can represent...with total black usually being represented internally by the number zero, and total white being represented by some finite maximum such as 255 (8bit), 4096 (12bit), 16384 (14bit), or possibly as high as 65536 (16bit) on the most recent and top of the line cameras and monitors.

This range is considerably more limited than the possible range of light intensity in the real world, by a factor of over 1500 times. When exposing a photograph, one has to be aware of the limited dynamic range. Expose too long, and you risk capturing more light than is possible to represent in 8-16 bits of information...at which time, you clip any excess analog value to the maximum possible digital value. In the photograph you have posted, it appears that the collar of the womans top has been overexposed, resulting in it being clipped. Her hair also appears to be a bit under-exposed, and while its not possible to expose less than 0, it is possible to expose too little, such that the electronic noise of the sensor itself overwhelms any useful image data.

It is possible to determine before capturing a photo with a digital camera whether you might be clipping the whites (or highlights, as they are usually referred to), by using the histogram. The histogram is a simple diagram that plots how many of each tone (a level of intensity, ranging from zero to maximum) is present in the photograph. The histogram normally progresses from left to right (however some cameras are opposite), with the darkest tones to the left, midtones in the center, and highlights to the right. If you are over exposing, the right-most tones will be maxed out...reaching the top edge of the histogram. When you see such a histogram, adjust exposure downwards until the highlights are flat or just begin to rise near the right edge. It should be noted that if you expose for the highlights, you might lose proper exposure elsewhere. Some cameras contain built-in modes, such as Canon's Highlight Tone Priority, that will attempt to preserve highlights automatically without radically changing the rest of the image, which might be of some use.