Short answer: Green OA is delivered by repositories and gold OA is delivered by journals.
With green OA, the contents can be embargoed or unembargoed; refereed (peer-reviewed) or unrefereed; previously published or not-yet-published; gratis (merely free of charge) or libre (gratis plus free for reuse under open licenses).
With gold OA, the journals might charge publication fees (also called article processing charges or APCs) or they might not. Gold OA is almost always peer-reviewed and unembargoed. It's easier for gold OA than for green OA to use open licenses, but most OA journals still do not use open licenses.
Green and gold OA are compatible in the sense that the same article, and even the same version of the same article, can be both at the same time. When well-implemented, both are entirely lawful. In longer pieces (links below) I've argued that green and gold OA are complementary, and that there are reasons to want each one even when we already have the other.
For more detail, see my Open Access Overview < http://bit.ly/oa-overview
For still more detail, see my book, Open Access (MIT Press, 2012) < http://bit.ly/oa-book
>, Chapter 3 ("Varieties") < http://goo.gl/VqYyqZ
>, and the updates and supplements to Chapter 3 < http://bit.ly/oa-book#ch3