Lots of articles are freely available online. Does that mean they are open access? Nope.
Here’s a nice post on the difference between public access and open access:
http://info.hsls.pitt.edu/updatereport/?p=6481.
Basically, the difference is in copyright ownership:
“In public access, funding agencies make research results freely available, while still retaining traditional copyright restrictions. As with a published journal article, anyone can read the findings, but no one may redistribute the content without permission. In OA, the content is freely available and may be reused or even republished by others without having to gain permission. Many OA authors choose to protect their content by applying a Creative Commons or similar license to their work.”
For more information, read the original post.