The directory emu contains the source and object code for the XSB emulator, which is written in C.
The directories syslib, cmplib and lib contain source and object code for the basic Prolog libraries, the compiler, and the extended Prolog libraries, respectively. All the source programs are written in XSB, and all object (byte code) files contain SLG-WAM instructions that can be executed by the emulator. These byte-coded instructions are machine-independent, so usually no installation procedure is needed for the byte code files.
The directory packages contains the various applications, written in XSB, which are not part of the system per se.
You must already be familiar with the build directory, which is what you must have used to build XSB. This directory contains XSB configuration scripts. The directory etc contains miscellaneous files used by XSB.
The directory docs contains this manual in LATEX , dvi and Postscipt format, and the directory examples contains sample programs to demonstrate various features of XSB.