A column or pillar is a large round support with a capital and base and made of stone. A column is a compression member, ie. it is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. Columns frequently support beams or arches. Many columns form part of a wall. A pillar is a tall and slender freestanding vertical support often used for decoration.
A post is an upright column or pillar. Generally, in architecture, post and lintel (prop and lintel) refer to a system where a horizontal member (lintel) is supported at its ends by two vertical columns or posts.
A pier is an upright rectangular support for a structure or superstructure, such as an arch or bridge. The lower portion of a pier may be widened to better distribute the downward pressure of a massive overlying structure.
A pilaster is a slightly projecting rectangular column built into or applied to the face of a wall. A pilaster is an ornamental and structural column. It is an upright architectural member that is rectangular in plan. A pilaster is a rectangular support that resembles a flat column. Structurally it is a pier but architecturally it is treated as a column. Pilasters are sometimes used to support fences and gates. They are also used to enclose mailboxes and light fixtures or to hold a planter or an ornament.
A pedestal is an architectural support, or base for a column, statue, vase, or obelisk. Pedestals may be square, octagonal, or circular. A single pedestal may support a group of columns. Display pedestals showcase art or plants or sculptures.