There have been five stand-alone† editions of Savage Worlds:
- Savage Worlds was printed in 2003 and was the first edition.
- Savage Worlds (Revised Second Edition) was printed in 2004.
- Savage Worlds Explorer's Edition was printed in 2007 and brought a significant rules revision. This also brought a smaller form factor and a low price, with the intention of making it an easy sell to curious GMs and their doubting players.
- Savage Worlds Deluxe was launched in 2011. It is an expansion on the rules in SWEX and contains minor rules revisions, but is intended to remain compatible with (and more importantly, not invalidate) existing material or SWEX. The most important changes relate to rules for injuries and incapacitation, damage from area of effect weapons and a change to the Shaken mechanic. This is also higher page-count hardcover, so it's more expensive.
- Savage Worlds Deluxe Explorer's Edition was launched in 2012 and is just the previous edition in a smaller, easier to carry around and cheaper format (same format as SWEX). However, it does have the errata from the larger Deluxe version included.
So the most up-to-date version is Savage Worlds Deluxe, but this is more a new version of the SWEX edition of the rules than a new edition. In the same way, the original Savage Worlds and Savage Worlds (Revised Second Edition) were mostly rules-compatible, so there have really only been two major editions of the rules, regardless of the number of books they've been contained by.
If you're using SWEX, you're using the most recent edition. This is the edition that all of the settings (first-party and third-party) and the Companions line of books are based on, and the Deluxe book this year indicates they're invested in keeping this rules base stable for players and third-party publishers for a while yet.
† Savage Worlds was evolved out of the rules for Deadlands and the Savage World of Solomon Kane, so it's arguable that there have been a couple of pre-first-edition editions of the rules as well.