The Paleozoic era was the first time in our planets history that multi-celled creatures became very prominent. Before these times, single celled bacteria were the primary forms of life on Earth. Changing conditions in earth triggered an explosion of multi-celled organisms that ultimately kickstarted the cycle of modern evolution. From many of these smaller organisms came larger fish and basic plants which eventually became what we know as modern life.
Even though life on earth was flourishing, every 10 to 100 million years, a mass extinction event occurred. During these events, a vast majority of life disappeared. These extinctions are one way we can tell the beginning and end of a period.