Though there might me a confusing point in Pagelayout and MasterPage, Difference lies in the processing of files and contents in SP.
Page Layout is first streamed and compiled with Content Page and then only the Associated Master Page is queried to SP File provider and again master page is compiled and a complete page is returned back to the Client or to the requesting Browser.
Page Layout is just an ASPX page which holds Page Content and you must know the mapping of Page Content and Page layout. But, master page is the same master page which you use on asp.net
Page Layout is first streamed and compiled with Content Page and then only the Associated Master Page is queried to SP File provider and again master page is compiled and a complete page is returned back to the Client or to the requesting Browser.
Page Layout is just an ASPX page which holds Page Content and you must know the mapping of Page Content and Page layout. But, master page is the same master page which you use on asp.net