I was trying to change / edit the master pages for a site.
 but for all team site the Masterpage was not being editable/changebale.
then i tried creating a Publishing site.
I tried openig  that site using SPD and bingo the master page
was visible for any site page  on the publishing site.

Publishing site :
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Publishing pages are available only in publishing sites. In publishing sites, authors and approvers use the publishing feature to create content and then make it available for site visitors. Usually, a publishing site has an approval workflow enabled, so content is reviewed and approved prior to being published.

Publishing pages are created by using page templates called page layouts. The page layouts provide a consistent structure to all publishing pages in a site collection. Page layouts can be customized to contain fields contributors can use to enter their content, such as text, graphics, rich media, and more.

Publishing pages are the most structured of SharePoint content pages, both in terms of their layout and the process for using them. An organization or department uses a publishing site to ensure that authors follow a business process to release content in a consistent manner. Press releases are a good example of a type of content that’s ideal for a publishing site.