RGB
The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. The very idea for the model itself and the abbreviation "RGB" come from the three primary colors in additive light models.
Note that the RGB color model itself does not define what exactly is meant by "red", "green" and "blue", so that the same RGB values can describe noticably different colors on different devices employing this color model. While they share a common color model, their actual color spaces can vary considerably.
YCbCrYCbCr is a family of colour spaces used in video systems. Y is the luma component and Cb and Cr the chroma components. YCbCr signals (prior to scaling and offsets to place the signals into digital form) are created from the corresponding gamma-adjusted RGB (red, green and blue) source using two defined constants Kb and Kr.
CMYK
CMYK (or sometimes YMCK) is a subtractive color model used in color printing. This color model is based on mixing pigments of the following colors in order to make other colors.
The mixture of ideal CMY colors is subtractive (Cyan, Magenta and Yellow printed together on white result to black). CMYK works through light absorption. The colors that are seen are from the part of light that is not absorbed. In CMYK magenta plus yellow produces red, magenta plus cyan makes blue, cyan plus yellow generates green and the combination of cyan, magenta and yellow form black.
The CMYK article includes a nice howto for converting to and from RGB.
Gamut
In computer graphics, the gamut, or color gamut, is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. Another sense, less frequently used but not less correct, refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process.
In color theory, the gamut of a device or process is that portion of the visible color space that can be represented, detected, or reproduced. Generally, the color gamut is specified in the hue-saturation plane, as many systems can produce colors with a wide range intensity within their color gamut; in addition, for subtractive color systems, such as printing, the range of intensity available in the system is for the most part meaningless outside the context of its illumination.
When certain colors cannot be displayed within a particular color model, those colors are said to be out of gamut. For example, pure red which is contained in the RGB color model gamut is out of gamut in the CMYK model.
F-stop
In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. For example, f/16 represents a diaphragm aperture diameter that is one-sixteenth of the focal length (or the focal length is 16 times the aperture).
The greater the f-number, the less light per unit area reaches the focal plane of the camera film.
In the case of a telescope instead of a camera, one may use a photovoltaic detector or other photometer device, a charge-coupled device (CCD), or the viewer's own eye as a substitute for film. In all cases, the principle is the same: the greater the focal ratio, the fainter the images created (measuring brightness per unit area of the image).
f-stops are a way of representing a convenient sequence of f-numbers in a geometric progression. Each 'stop' is marked with its corresponding f-number, and represents a halving of the light intensity from the one before, corresponding to a decrease of the diaphragm aperture diameter by a factor of √2, and hence an halving of the area of the aperture.
