Digital camera image sensors consist of a number of pixels, which collect photons (energy pockets of light). The photodiode then converts the photons into an electrical charge. Each pixel has only one photodiode.
A conventional sensor in, for example, a 12MP (megapixel) camera has an almost equal number of effective pixels (11.9MP). Therefore, effective pixels refers to the area of the sensor that said pixels can cover. The 0.1% of pixels left are used to determine the edges of an image and to provide color information. On occasions, not all sensor pixels can be used (for instance, if a lens cannot cover the whole sensor range).
Some cameras can interpolate the number of sensor pixels. For instance, a 6MP camera can produce 12MP images. Here, the camera interpolates 12 megapixels of information based on the 6 megapixels of effective pixels on the sensor. (The term interpolation refers to the process of making new pixels, based on the captured pixels.)
Interpolation cannot ever create data that you didn't capture in the first place, and the difference in quality is marginal.