Page layout is the part of graphic design that deals in the arrangement and style treatment of elements (content) on a page.
A page layout is part of the web content management publishing model in SharePoint. The page layout defines the design of one or more publishing pages which is decoupled from the actual content. It controls how the content is rendered by means of field controls, static content and controls, and web part zones. Questions having the page layout tag refer to the design and implementation of page layouts in publishing sites.
More to read: http://msdn.microsoft.com/en-us/library/ms543497.aspx