A 'color space' is the term used to describe a system of specifying colors. There are quite a number of different types of color space in common usage, and there are quite a number of variants of each type too.
When a color space with no colorimetric interpretation is encountered, then some external definition must be supplied, or else the meaning of the color information may be either unknown, or become altered on translation from one system to another.
Two commonly encountered color spaces are:
RGB which represents colors using Red, Green and Blue luminous additive components.
This type of color space is typically used by display screens and is also popular with digital cameras and scanners. Images stored in BMP, PNG, JPG and some TIF files often use this.
CMYK which represents colors using Cyan, Magenta, Yellow and blacK ink subtractive components.
This type of color space is typically used by color printers. Images can be stored using this color space in TIF files.