Are dinosaurs classified as reptiles or are they in their own class?
from Howie P., age 33, Efland, NC, USA; July 7, 2001

TOM: Biologists have been replacing the old Linnean style rank-based classification (with its Classes and Orders and such) with a newer system that is focused on evolutionary history rather than "ranks". Many of the names used were ones that had previously been assigned a particular "rank", but in the new style of classification they represent clades (complete branches of the Tree of Life; in other words, all descendants of a common ancestor).

Because it focuses on clades, this new style of classification is called "cladistics". Clades represent whole groups, but even within this group their can be all sorts of evolutionary changes.

So Dinosauria is part of a larger clade, Reptilia. However, Aves (birds) is a clade within Dinosauria, and thus a clade within Reptilia. This means that birds are technically a type of reptile.