A favorite term by those in the industry, a "walled garden" refers to certain Web sites or OSPs (Online Service Providers) that give their members easy access to their own content (and their partners' content) but no access to the content of their competition. This is in contrast to most other Web sites or ISPs (Internet Service Providers) that give users open access to all Internet sites and content.
The most famous early example of a walled garden is AOL. More recently some social networking sites such as Facebook, are also considered walled gardens due to the fact that the information and photos you post on Facebook are only accessible by other members of Facebook.
Here is an example of its usage, as quoted from the blog of Scott Heiferman (Founder of meetup.com). He says "While at Sony in 1994, I was sent to Virginia to learn how to build a Sony app on AOL using AOL's proprietary rainman platform. Fast forward to Facebook in 2007 and you see similarities: If you want access to their big base of users, you need to develop something in their proprietary language for the people who live in their walled garden."
NetLingo Classification: Online Jargon