Classification is a hierarchical scheme: the highest grouping is the phylum. Within a phylum, organisms are divided into classes (groups outlined within the two subphyla of the echinodermata are classes - the echinoids belong to the Class Echinoidea). A class is further divided into orders, orders are divided into families, families into genera, and genera into species.
Classifications now are constructed according to the principles of cladistics. Organisms are grouped in a hierarchical manner according to shared characteristics, which are taken to indicate common descent. Monophyletic groups are those whose members are all descendants of a common ancestor. Polyphyletic groups contain members that arose from more than one ancestor.
The geological record is divided up into periods - these are assigned names which may be used instead of absolute numbers. Indeed, the naming arose when absolute dating methods did not exist. The earliest period is seen at the bottom of the table - the Cambrian. Going up the list becomes more and more recent in geological time, though for example the boundary between the Cretaceous and Tertiary was approximately 65 million years ago. These periods are grouped into eras: the Palaeozoic, Mesozoic and Cenozoic. In much of the description of this site the Palaeozoic and post-Palaeozoic are referred to - the 'post-Palaeozoic' includes both the Mesozoic and Cenozoic.