Swords & Wizardry is a well-known retroclone of the original D&D rules produced under the aegis of the OGL. The Whitebox version of the rules seeks to reproduce the game experience of the original edition of the original D&D booklets with minimal rules. This publication is the third print edition of the Swords & Wizardry Whitebox Rules.
designer's commentary from Dragonsfoot forums:
Various Modifications in this printing:
Now includes the saving throw categories (as an alternate, for legal safety reasons, but the numbers are there)
Now includes a closer model of the original XP bonus system (also labeled as "alternate")
An alternate sequence of combat divides the combat round into separate "phases" - this interpretation of CM is very close to Holmes Basic.
Marv's universal attribute bonus is now labeled as an optional system, to clarify that it's not part of the original rules.
Chaos and Law are more integrated as the "default" alignment system, to link up a bit better with the other places where alignments are mentioned (mainly spells).
No neutral clerics, chaotic clerics cannot turn the undead (all as per straight 0e)
This printing essentially adds options that bring it closer to the original, but they don't change anything that's already existing, so there's no reason to re-purchase the book. I have ordered my test copy of the print book from lulu - it is a 6x9 hardback - and once I make sure that they haven't turned the pdf into garbage, I'll put it up for general availability as both hardback and softcover.
This edition of the rules also features different layout, artwork and explanatory sidebars. Another difference is the credited authors, Matt Finch and Mike Shorten (chgowiz), and an expanded list of editors.