The 2.0.x series of Geotools represents a major restructuring and improvement over the 1.0 series of Geotools.
This series has now been retired, with the last stable release from this branch being 2.0.0.
This release featured all manner of improvements over GeoTools 1.0, and represents the community aligning itself with open standards. A major component of this effort was refactoring the code base into a new modular structure, as desribed here.
The original developers guide is available: