Swords and Wizardry: Core Rules is a retroclone of the original fantasy role playing game by Gary Gygax and Dave Arneson.
Core Rules is an OGL compliant recreation of the original three booklets plus a selection of rules from the supplements.
This edition of S&W: Core Rules features rules changes, new artwork and layout.
designer's comments about differences:
Mainly additional alternatives or more resources, not underlying changes. As examples, (a) the full saving throw matrix is included as an alternative for those who don't like the single saving throw category, (b) a wilderness encounter table is included, (c) there are some more monsters, (d) attribute effects and hit dice comport with Supplement 1 instead of being a fusion with WhiteBox, [this is where the changes are - polyhedral HD instead of modified d6, open door rolls, etc],(e) there is a alternative sequence of battle similar to that of the Holmes blue book, (f) there is a two-page sample campaign setting (using my Mythrus Tower campaign). In-combat movement rates fixed/clarified. Various errata, mostly grammatical or layout. I think that's about it. Some new art. Sample dungeon mapping symbols, different sample dungeon map.