The pixels you get in a finished, camera-independent image file - a JPEG, TIFF, BMP, ...) file you get either from your camera or from your RAW processor software - are usually the results of multiple photodiode outputs (sensor pixels) combined, due to the way color filters arrays work.

The pixels in an image file, or in an image displayed on a monitor, are RGB pixels which each have 3 color channels to themselves, whereas a sensor pixel from a color digital camera will represent only one color channel - processing in camera or computer will interpolate the other color channels from adjacent sensor pixels following a so called demosaicing algorithm. You do get the same amount of output pixels as you have sensor pixels - the luminance (brightness) resolution of the camera will still be equivalent (unless limited by an AA filter, see below) to the nominal resolution of the sensor, whereas the color resolution is actually somewhat lower.

This does not affect the output as much as one would assume as long as one does not photograph extremely fine tartan-like patterns of contrasting colors, or subjects made of pure color noise. This could result in so called moire effects or other color artifacts.

The actual resolution of many sensors is actually optically limited by so-called antialiasing (AA) filters. This is because even a pure luminance sensor (black&white) can actually only resolve as high as its nominal resolution under optimal and very unreliable conditions: If you projected a checkerboard pattern with fields equivalent in size to sensor pixels on to a sensor, you could end up with a perfect reproduction of the pattern... or with a perfect uniform gray, if the sensor was misaligned 1/2 pixel wide with the checkerboard pattern. Conservatively (following Nyquist's sampling theorem) you would have to cut the resolution by half in each dimension (making a 6MP sensor of a 24MP sensor); in practice, compromises are made (The filter will attenuate contrast progressively the closer you get to theoretical resolution),or the AA filter is omitted, either leaving it to software to recognize such artifacts and correct them, or simply accepting the risk.