Open Access is a term that describes how publishers or authors make content freely available. There are different levels of "open," commonly categorized by color.
GOLD: Complete open access
GREEN: Can archive pre-print and post-print or publisher's version/PDF
BLUE: Can archive post-print or publisher's version/PDF
YELLOW: Can archive pre-print
WHITE: Archiving not formally supported
Pre-print: A first draft, prior to peer-review
Post-print: A final draft, with peer-review incorporated but without journal formatting
To self-archive is to deposit a digital document in a public repository. As the author, you are asked to provide basic metadata about the document—author name(s), title, publication date, journal title, etc.—so that it can be properly indexed. The repository will review the metadata and make sure it is formatted properly and upload the document to the web.
No, the copyright holder retains copyright of the work. This may be the author(s) or the publisher, depending on the original publishing contract, or the version that you choose to self-archive. The resources for authors section has more information.
Once your work is in a public repository, the full text of the document is now visible, accessible, harvestable, searchable, and usable by anyone with an Internet connection. It also ensures your work will remain publicly accessible and preserved for the long-term.
Self-archiving your work (whether it is published articles, unpublished documents, or supplementary data) can be accomplished in several ways. One of the most common methods is to license the work under a Creative Commons license. Many repositories and open access journals use this method, as CC licenses are free, easy to use and understand, and standardized.