"Beta" and "preview" usually mean the same thing. The only difference in terms of common usage is that many (not all) beta versions of software are closed--for example, you will never (legally) see a beta version of Photoshop floating around for all and sundry to use, they're kept close by Adobe--whereas a preview release is equivalent to something like a "public beta" (an early version that anyone can see).
A release candidate (RC) version is a version that's very close to being finalized. It's still beta, but unlike a normal beta, it's feature-complete; the author is not planning to make major feature changes, or interface changes, or anything like that. At this stage, it's bug-fixes only. Sometimes, the released version of a piece of software is identical to the last release candidate version.