Explicit Multi-Threading (XMT) is a computing framework developed at the University of Maryland as part of a PRAM-on-chip vision (http://www.umiacs.umd.edu/users/vishkin/XMT). Much in the same way that performance programming of standard computers relies on C language, XMT performance programming is done using an extension of C called XMTC. This manual presents the second generation of XMTC programming paradigm. It is intended to be used by an application programmer, who is new to XMTC. In the first part of this technical report (UMIACS-TR 2005-45 Part 1 of 2), we define and describe key concepts, list the limitations and restrictions, and give examples. The second part (UMIACS-TR 2005-45 Part 2 of 2) is a brief tutorial, and it demonstrates the basic programming concepts of XMTC language with examples and exercises.