The Precambrian Era was originally designated as all the rocks below or older than the Cambrian. These rocks, older than 544 million years, were originally lumped together due to their relative lack of fossils. More recently the era has been subdivided into three time periods based on radioactive dating and the discovery of numerous fossil locations. The subdivisions are:
the Hadean - 4,600 to 3,800 million years ago,
the Archaean - 3,800 to 2,500 million years ago and
the Proterozoic - 2,500 to 544 million years ago.
The Precambrian Era spans over three billion years,whereas the time since the end of the Precambrian has onlybeen 500 million, or a half billion, years. This covers the time from the "birth" of the earth to what is known as the "Cambrian explosion" when fossils became abundant in the rocks.
They are located below the rocks of the Cambrian