SharePoint master pages provide the interface and overall layout of the pages on a SharePoint site.The master page displays the persistent elements that you expect to see when you navigate from page to page on a site. These elements can include a company logo, title, navigation menu, search box, and the colors or graphics associated with the business or organization.
A master page serves as a container for all of the parts of a content page, and it also contains the necessary code to render the SharePoint interface. You change the user interface or the brand of a SharePoint site by customizing the master page. You can customize the out-of-the-box master page or create a new master page using design manager.
In SharePoint 2013 there are 2 default master pages, rather than only the 1 that was available in SharePoint 2010. The default master pages in SharePoint 2013 are named
The site master page is used by all publishing pages, or pages that visitors to your website can see. It allows for the customization of publishing pages differently than system pages, and also allows for a different master page to be displayed to different devices using device channels. It is not used by any other page by default. Though it is possible to edit the master page using SharePoint Designer, it is not recommended unless absolutely necessary.
The system master page is used by SharePoint everywhere the site master page is not, including administrator pages, lists, and document library views. Be very careful customizing this page as it is used extensively throughout all SharePoint sites.
A better editing experience for Microsoft SharePoint Foundation pages
Developers can better use their time and resources by making design changes in one place and the changes will be reflected in all pages that use the master page.
User interface reuse
Users can easily create a page with the SharePoint Foundation look and feel. By referencing the default master page, a page that is based on it gets the Microsoft SharePoint Foundation look and feel and also picks up any updates to the master page.
More consistent pages and a better end-user experience
Because all pages that are associated with a master page have the same consistent look and feel, end users can browse from a core Microsoft SharePoint Foundation page to a page added by a third-party solution or to a page that has been customized by the Web designer, and not notice a difference in the way the site looks or in how its controls work.
There is a new feature in SharePoint 2013 that allows for the creation of a new master page from an existing HTML page. This is a drastic change from master page customization in SharePoint 2010 and has been a huge relief to SharePoint designers and power users.