"Open Access" generally refers to scholarship that is free to read and re-use.
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.
There's a lot of misinformation out there about Open Access. People believe OA publications aren't peer-reviewed, have low impact factors, and require author fees. The truth is that while there are some poor quality OA publications, there are several high quality, high impact, well respected OA publishers as well. Some of them require author fees, but most do not. It's important to remember that both of those statements could also be made about subscription publications. Here are a few links to some OA myth de-bunking articles:
These are a few of the most notable OA publishers: