Open Access generally means "available freely to the public via the Internet," which the Budapest Open Access Initiative defines as "permitting any users to read, download, copy, distribute, print, search, or link to the full texts of these articles, crawl them for indexing, pass them as data to software, or use them for any other lawful purpose, without financial, legal, or technical barriers other than those inseparable from gaining access to the internet itself."
Generally speaking, there are two approaches to providing open access:
1) Self-archiving involves the direct deposit of scholarly works into an open repository such as DRUM. It is not a publishing method and its express purpose is to make information as accessible as possible. Self-archived works may be published, to-be-published (pre- and post-prints), or unpublished (many theses and conference papers). This route to open access is known as the Green Road.
2) Publishing in open access journals allows full text access for free. This route is called the Gold Road and complements - does not replace - the Green Road.
Delayed Open Access: Journal articles are freely available after a period of paid access, anywhere from 6 to 12 months, expires.
Short-term Open Access: Free access to journal articles is provided for a short time after publication, after which access is only available to subscribers.
Selected Open Access: Selected journal articles are freely available, while the remaining articles are accessible by subscription.
Hybrid Open Access: Authors are given the option to pay a publication fee to make their article freely available immediately on publication.
Partial Open Access: The primary research articles of a journal are made freely available, but access to other value-added content such as editorials and review articles requires a subscription.
Total Open Access: All the articles in a journal are free and accessible on the Internet. Article processing fees are usually required to cover the costs of peer-review and online publication and are paid by the author, the author's institution, or the author's research grant. Many open access journals offer institutional memberships where, based on the level of membership, article processing fees are either reduced or waived.
Andrew Bonamici's (U of Oregon) blog post: Avoiding Scams.
Butler, Declan. "Investigating journals: The dark side of publishing." Nature 495, no. 7442 (27 March 2013). Includes a checklist to identify reputable publishers.
Jeffrey Beall's Criteria for Determining Predatory Open-Access Publishers. Also includes a list of "predatory" publishers and individual journals.