Chromatography is the name of a wide category of methods for separating components of mixtures. Chromatography was invented by a Russian botanist named Mikhail Tswett around the beginning of the twentieth century. He separated plant pigments by putting solutions of these compounds through glass columns packed with calcium carbonate. The separated pigments formed different bands of color as they moved through the column, explaining the use of chroma (color) in the name he gave to the technique.
All chromatographic methods involve introducing a sample into a mobile phase (usually a liquid or gas, but more exotic things such as supercritical fluid can also be used) which is forced through an immiscible stationary phase which is either bonded to a solid surface or packed into a column. As the mobile phase flows through or over the stationary phase, different compounds will flow through at varying rates based on how strongly or weakly they are retained by interactions with the stationary phase. Separations are made possible because of the different properties of substances in a mixture, such as different ionic strengths, affinities, adsorption properties, or even size. If the mobile phase and/or stationary phase is not chosen carefully it is possible for a compound to be "unretained", that is, it flows through the stationary phase at the same rate as the mobile phase. It is also possible for a compound to be completely retained on the stationary phase, never eluting no matter how much mobile phase is used.
Some of the more common types of chromatography include column chromatography, high-performance liquid chromatography (also known as high pressure liquid chromatography or HPLC), gas chromatography (GC) and thin-layer chromatography (TLC).