"Open Access" (OA) generally refers to scholarship that is free to read and re-use.
SPARC's definition of Open Access.
Peter Suber's definition of Open Access.
When it comes to making your work OA, there are two different ways to do this:
Green OA: Publish in a subscription journal but keep your rights and archive a copy of your article in an disciplinary repository. This allows authors to meet the public access requirements that many research funding agencies have.
Gold OA: Publish in an Open Access journal.
There are two other distinctions to keep in mind when talking about OA:
Libre OA: Scholarship that is free of most licensing and copyright restrictions.
Gratis OA: Scholarship that is free to read, but still has licensing and copyright restrictions.
These are a few of the most notable OA publishers: