Hi Kimkia - From the WebmasterWorld Glossary (there's a link to it at the top of your page)...
"Run of Site. An ad that can be placed anywhere on a website without restrictions.."
Something interesting I notice about this definition... it automatically assumes such links are "ads," an assumption that search engines undoubtedly make too.
For a discussion of the value of ROS links, see...
What is the Value of Run of Site Text Link Purchase?
How do you use this potential goldmine/ pitfall?