A mummy is simply a human being whose soft tissue has been preserved long after death. Ordinarily, when a person dies, the decomposition process reduces the body to a bare skeleton in a matter of months. The rate of decomposition is dependent on a number of factors, chiefly the nature of the surrounding environment. In most environments, the first stages of decomposition begin within a few hours.
Autolysis is followed by putrefaction , the breakdown of organic matter by bacteria .
In normal, temperate circumstances, putrefaction gets going about three days after death. Within a few months, the body is reduced to a skeleton. In hotter, more humid environments, this process is accelerated, because bacteria reproduce rapidly in such conditions. If the conditions are cold or dry enough, or if there isn't enough oxygen, the environment is so harsh that few bacteria can survive. In this case, the body will not fully decompose, possibly for thousands of years.