An order is the principal component of a Classical building, composed of base, shaft, capital and entablature.
The five Classical orders are the ‘Tuscan’, ‘Doric’, ‘ Ionic’, ‘Corinthian’ and ‘Composite’, which all vary in size and proportion.
The Tuscan, a Roman order, is the plainest and largest.
The Doric is found in two distinct varieties: Greek Doric IS characterized by its fluted shaft and lack of base; Roman Doric may have a fluted or unfluted shaft and also has a base. The Ionic is Greek in origin, but was used extensively by the Romans; it is distinguished by its voluted capital and often fluted shaft.
The Corinthian is characterized by the acanthus leaves decorating its capital.
The Composite is a Roman creation that combines the volutes of the Ionic with the acanthus leaves of the Corinthian.
Overall, all classical architecture of the Greco-Roman tradition is composed, or written, in one language of forms. These elements of classical architecture include specific Moldings and assemblages of moldings called an Order. An Order is an accepted way of assembling a column (supporting element) with an entablature (spanning element) while imparting a certain character. In short, an Order orders a design. Orders are never applied after the building is designed, as they are generative.