  The Cosmic Linear Anisotropy Solving System (CLASS) is a new accurate
Boltzmann code, designed to offer a more user-friendly and flexible coding
environment to cosmologists. CLASS is very structured, easy to modify, and
offers a rigorous way to control the accuracy of output quantities. It is also
incidentally a bit faster than other codes. In this overview, we present the
general principles of CLASS and its basic structure. We insist on the
friendliness and flexibility aspects, while accuracy, physical approximations
and performances are discussed in a series of companion papers.
