Two basic types of temples remain from the Khmer empire: the temple mountain and the monastic complex. 

An Example of Khmer Stone Carving (from Banteay Srei)

The temple mountain has a strong vertical emphasis. It is supposed to symbolize the mythical Mount Meru that in both Hindu and Buddhist mythology stands at the center of the world and separates the regions of earth from the regions of heaven and hell. In Khmer architecture the early form of the temple mountain  was a square tower on a tiered based, built on a natural or artificial hill. Later the architectural concept was developed into a multi-platform, five tower arrangement, often surrounded by a moat, as in Angkor Wat, for instance, where a central tower is surrounded by 4 smaller ones placed in the corners of the top level of a tiered base. (The most famous examples for this type of temple architecture are Angkor Wat, the Bayon, Preah Rup or Phnom Bakheng). 

The monastic complex type of temple, on the other hand, was laid out on a horizontal plane. There is no difference in height between the entrance and the central shrine. It is surrounded by an outer enclosure wall and inside space is defined by one or more concentric walls that separated the living quarters of the religious community from the temple proper. Additional walls divide the monastic complex into courtyards and arcades. Two lovely temples of this type are Preah Khan and Ta Phrom.

Typical elements of Khmer Architecture

Gopura

Gopuras (the term comes from south Indian temple architecture) are gateways that lead into a temple; they are placed on the principle axes of the enclosure wall. Early Gopuras are simple and rectangular; the later forms are very elaborate and look like separate buildings with porches and steps.

Causeway

Many Khmer buildings were surrounded by a moat; causeways led across the water to the main complex. Very often the causeway has a Naga balustrade (nagas are mythical serpents).

Enclosure wall

A typical feature of the temples at Angkor. They are quite massive (mostly built of coarse laterite stone) and archeologists believe that their purpose was not so much to provide physical protection, but a psychological barrier between then profane world outside and the sacred world inside. 

The famous causeway to Angkor Thom

Sikhara

Like "Gopura" this is a term derived from South Indian temple architecture. Sikharas are towers, of course the most prominent feature of Khmer architecture. They stand on platforms and have open or false doorways on each facade. Khmer sikharas have a rounded top and are crowned with a stylized lotus bud which probably supported a golden or gilded spire (e.g. as in today's Thai temples). The outer surface is usually beautifully carved.

Pavilion (Shrine)

Smaller building inside the temple, often found in pairs along the axial road leading to the sanctuary (e.g. the "libraries" at Angkor Wat).

Corbelled Vault

A simple type of arch. In Khmer buildings, passageways and galleries were covered with corbelled vaults (an overlapping arrangement of blocks of stone). Unfortunately, the corbelled vault is a very unstable type of arch.