Liquid crystal displays (LCD) use a grid of definable points on a screen to display information. Used in many of Apple's products, including Cinema Displays, MacBooks and MacBook Pros, and the iPhone, iPod Touch, and iPad, LCD screens can be subject to pixel anomalies. As Apple puts it:
Each pixel has three separate subpixels--red, green and blue--that allow an image to render in full color. Each subpixel has a corresponding transistor responsible for turning that subpixel on and off.
If any of these subpixels or transistors fails, it can result in a "dead" pixel on your display. Pixels generally number in the millions, depending on the size of your display. Apple's 27-inch iMac, for example, has a display resolution of 2,560x1,440, resulting in 3.7 million pixels.
Once you take into account the subpixels, you're talking about over 11 million elements involved in displaying images on your iMac.
Pixel anomalies can present themselves in several different ways. Occasionally, dust or other debris can appear to be a dead pixel on an LCD screen. Though Apple is generally one of the best quality-control companies out there, the sheer volume of displays the company produces can result in anomalies. According to Apple:
Foreign material is typically irregular in shape and is usually most noticeable when viewed against a white background. Foreign material that is on the front surface of the glass panel can be easily removed using a lint free cloth. Foreign material that is trapped within the screen must be removed by an Apple Authorized Service Provider or Apple Retail Store.
Pixels that have actually failed may appear as either dark spots or bright spots, depending on what's wrong with them. Apple's pixel anomaly replacement policy was recently leaked, revealing how many anomalies need to appear on a display for Apple to consider replacing it.