Cambrian explosion refers to period of geological history of our planet in which most of the multicellular organisms appeared in fossil layers. That was some 540 million years ago.
This period indicates that multicellular organisms began to develope in the early Paleozoic era. Before that much of the living organisms were singlecellular. Cambrian is the first period of Paleozoic era and it lasted between 540 and 485 million years ago. It is followed by Ordovician Period.
The bottom line is that Cambrian explosion is related with 'great occurrence of new and developed species'.
