On the Internet, a walled garden refers to a browsing environment that controls the information and Web sites the user is able to access. This is a popular method used by ISPs in order to keep the user navigating only specific areas of the Web, whether for the purpose of shielding users from information -- such as restricting children's access to pornography -- or directing users to paid content that the ISP supports. America Online is a good example of an ISP that places users in a walled garden.
Schools are increasingly using the walled garden approach in creating browsing environments in their networks. Students have access to only limited Web sites, and teachers need a password in order to leave the walled garden and browse the Internet in its entirety.
Definition taken from http://www.webopedia.com/TERM/w/walled_garden.html