In the standard, it’s just "transform". "-webkit-" is called a "CSS vendor prefix" and the idea is this: If the standard changes before it’s finalized — it does happen — but Safari just implemented "transform" and not "-webkit-transform", then sites created in the interim could be stuck with broken CSS, retroactively. Mozilla did this forever ago with "-moz-border-radius" rounded corners. Even Microsoft started using the "-ms-" vendor prefix.
Apple using "-webkit-" is actually preserves the "open" and "standards" parts of "open standards". Otherwise it’s embrace and extend. (http://en.wikipedia.org/wiki/Embrace_extend_and_extinguish)
Edit: For clarity, this is how things like -webkit-transform are born (slightly simplified):
1. Apple (or Mozilla, or Opera, or Microsoft) decides it would be nice if the web stack could do X.
2. Apple devises a property and syntax, implements it internally, evaluates performance hits, iterates, etc.
3. Apple releases a public developer build (a "nightly") with X, using the -webkit- prefix, and blogs about it. http://webkit.org/blog/130/css-transforms/
4. Apple drafts a spec (no "-webkit-") and submits it to relevant standards bodies
5. Standards bodies, browser makers, and document authors weigh in
6. The draft is modified, clarified, dropped, and/or implemented elsewhere. http://webkit.org/blog/130/css-transforms/#comment-23010
7. The draft becomes more or less finalized, and user agents should support X without a vendor prefix