From The Art and Popular Culture Encyclopedia
A classical order is one of the ancient styles of building design in the classical tradition, distinguished by their proportions and their characteristic profiles and details, but most quickly recognizable by the type of column and capital employed. Each style also has its proper entablature, consisting of architrave, frieze and cornice. From the sixteenth century onwards, theorists recognized five orders.
Ranged in the engraving (illustration, right), from the stockiest and most primitive to the richest and most slender, they are: Tuscan (Roman) and Doric (Greek and Roman, illustrated here in its Roman version); Ionic (Greek version) and Ionic (Roman version); Corinthian (Greek and Roman) and composite (Roman). The ancient and original orders of architecture are no more than three, the Doric, Ionic and Corinthian, which were invented by the Greeks. To these the Romans added two, the Tuscan, which they made simpler than the Doric, and the Composite, which was more ornamental than the Corinthian.
The order of a classical building is like the mode or key of classical music. It is established by certain modules like the intervals of music, and it raises certain expectations in an audience attuned to its language. The orders are like the grammar or rhetoric of a written composition.