Here are some links that may be useful in understanding the D0 run II software.
D0 at work - Home page for D0 collaborators
Computing - Home page for D0 computing
Code management - Home page for D0 code manageme nt
The code - directory leading the the released D0 code (see releases/...)
Algorithms - Home page for D0 reconstruction and its algorithms
Framework - The framework provides access for reading and writing event data
RCP - RCP system allows users to provide input data to control execution
PAW - Standard analysis package for HEP data (superseded by ROOT?)