A column is a vertical member consisting of base, round shaft, and capital designed to carry an arch or entablature but also used non-structurally. The column with the entablature it supports was referred to as an “order” in classical architecture. Each order consists of its characteristic column, with base below and capital above, supporting its appropriate entablature of three parts: architrave, frieze (the widest part), and the projecting cornice above. The order was the unit of a colonnade (range of columns). Orders were first classified by Marcus Vitruvius Pollio in the first century B.C. in De Architectura Libri Decem, a treatise on building types, town planning, and the training of architects.

The Greeks originated the three basic kinds of orders: the Doric, Ionian, and Corinthian.  The proportions, moldings, and formalized ornament of the three orders are all varied to form a basic vocabulary of design which was later to be taken over and developed by the Romans as part of their more complex and ambitious architectural systems. The Doric order was the first one to be evolved over several hundred years and was composed of a sturdy tapering fluted column without a base but with a plain molded capital, and an entablature the frieze of which is divided into triglyphs (a block with three vertical grooves or channels) alternating with square panels, often carved.  The Ionic order with its characteristic angle volutes (scrolls) on the capitals was derived from the Greek architecture of Asia Minor.

The Romans added the Tuscan Doric, with its less sturdy and often unfluted columns, and later the Composite form, which combines the capitals of the Corinthian and Ionic.  In the Corinthian order, Roman practice differed from Greek in that the acanthus leaf decoration on the capital was carved from a different species of the plant and the column shafts were sometimes unfluted.  To give columns greater height the device of raising them on pedestals was frequently adopted.

An illustration of types of orders from A History of Architecture in Italy by T. W. West. Orders functioned as the basic grammar of classical architecture.

(The above summary is based on descriptions given in A History of Architecture in Italy by T. W. West.  This volume is highly recommended reading for individuals with an interest in the architecture of Italy.)