There are literally hundreds of methods in OpenCV that pertain to the conversion of color spaces. In general, three color spaces are prevalent in modern day computer vision: gray, BGR, and Hue, Saturation, Value (HSV).
Gray is a color space that effectively eliminates color information translating to shades of gray: this color space is extremely useful for intermediate processing, such as face detection.
BGR is the blue-green-red color space, in which each pixel is a three-element array, each value representing the blue, green, and red colors: web developers would be familiar with a similar definition of colors, except the order of colors is RGB.
In HSV, hue is a color tone, saturation is the intensity of a color, and value represents its darkness (or brightness at the opposite end of the spectrum).