The very short answer is, S&W is a "Retro-Clone" of the old, original Dungeons and Dragons game.
The complete rules are online at http://www.d20swsrd.com/, and you can buy printed copies from the same site. The same site gives a good description of S&W's goals and history. Much like their Pathfinder sister site, the d20swsrd is an invaluable resource for search for rules and such. Also, the downloads section contains character sheets, adventures and other aids.
"Why not just play the old D&D?" you ask? S&W has cleaned up many inconsistencies and vague wordings, and has added optional rules to smooth over some of the clunkier aspects of the original D&D, such as the crazy "low is good" armor class system. S&W includes both that system and a more d20-ish "ascending AC" system, which is supported through all of their creature lists and such. For gamers like me who have lived through the "to hit" tables and THACO, I'm happy to have the modern system available, as in my opinion it is the "right way" to do AC.
And, more subjectively, I believe Matt Finch's (the author of S&W) writing style is very clear and engaging. I first read S&W when I got a copy from the Reaper Kickstarter, and reading it made me want to play so much that I ordered the hardback version from Frog God Games.
A nifty innovation of S&W is that there is only a single save number, which is used for all saving throws. The author did include the original save table as an optional rule, but the standard S&W save is a single number. Certain classes get bonuses for different kinds of saves, such as the Magic User's "+2 for saves against effects caused by spells". At first, I wasn't sure about this system, but the more I use it the more I like it. The old system was often vague (is a wand of petrification a paralyze/petrify or a wand save?). Also, others have suggested using the Save for any situation that requires a throw of some sort that isn't covered by another rule, since the save improves with level. I've used it for a "three strikes" type of death system, sort of like 4e's, where once you drop to zero hitpoints, you must make saves each round until you either pass three and stabilize or fail three and die. A friend can stabilize you as their round's action.
Well, that's it. If you're at all intrigued, head on over to the SRD site and see what you think.