Lustre was initiated and funded, almost a decade ago, by the U.S. Department of Energy Office of Science and National Nuclear Security Administration laboratories to address the need for an open source, highly scalable, high-performance parallel filesystem on then-present and future supercomputing platforms. Throughout the last decade, it was deployed over numerous medium-to large-scale supercomputing platforms and clusters, and it performed and met the expectations of the Lustre user community. At the time of this writing, according to the Top500 list, 15 of the top 30 supercomputers in the world use Lustre filesystem.This report aims to present a streamlined overview of how Lustre works internally at reasonable detail including relevant data structures, APIs, protocols, and algorithms involved for the Lustre version 1.6 source code base. More important, the report attempts to explain how various components interconnect and function as a system. Portions of the report are based on discussions with Oak Ridge National Laboratory Lustre Center of Excellence team members, and portions of it are based on the authors' understanding of how the code works. We, the authors, bear all responsibility for errors and omissions in this document. We can only hope the report helps current and future Lustre users and Lustre code developers as much as it helped us understanding the Lustre source code and its internal workings.