There are thousands of OA journals, but some journals and publishers are more open than others; some are hybrid offering select articles as OA, while others are less open again. See HowOpenIsIt for a good overview of the spectrum of publishing options.
The main types of open access are:
Gold: articles in fully open access journals, all article processing charges are paid for by the author
Diamond: articles in fully open access journals where articles are funded by the journal's parent body
Green: the author can "self archive" a version of the article in a repository or their personal website
Hybrid: a traditional subscription journal allows authors to pay to make their article open access
Open Access refers to unrestricted online access to articles published in scholarly publications. Types of open access publications available online include articles, books and book chapters, conference papers, theses, working papers, data and images.
There are three different ways of obtaining open accessibility to scientific research results:
Self-archive an open access version - Authors publish in the journal of their choice and archive or link to a freely available version of the manuscript in their institution's repository (UC Research Repository), or in a national repository (e.g. PubMed Central). A large percentage of publishers permit authors to archive a version of their article in an institutional repository.
Publish in an open access journal - Authors publish in Open Access journals that provide free and immediate access to the articles via the publishers web site. Authors may be required to pay an article processing charge.
Pay to publish open access in a traditional journal - a large percentage of subscription journals offer an Open Access publishing option, where articles can be made immediately available via open access. Authors are required to pay an article processing charge.
Introductory writings about open access by advocate Peter Suber