It is a vast open-air museum and the largest ancient religious site in the world, and is probably the second most visited ancient site in Egypt, second only to the Giza Pyramids near Cairo.
It consists of four main parts, of which only one is accessible for tourists and the general public. This is also the "main" temple part and by far the largest part. This is the Precinct of Amon-Re. One can probably on that basis redefine the term Karnak, as to be understood as the Precinct of Amon-Re only, as this is the only part most visitors normally see. The three other parts are closed.
The four main parts are;
In addition there are also a few smaller temples and sanctuaries located outside the enclosing walls of the four main parts, as well as several avenues of ram-headed sphinxes connecting the Precinct of Mut, the Precinct of Amon-Re and Luxor Temple.
The key difference between Karnak and most of the other temples and sites in Egypt is the length of time over which it was developed and used. Construction work began in the 16th century BC. Approximately 30 pharaohs contributed to the buildings, enabling it to reach a size, complexity and diversity not seen elsewhere. Few of the individual features of Karnak are unique, but the size and number of features is overwhelming.