What is the difference between SDS-PAGE and western blot?

SDS-PAGE is an electrophoresis method that separates proteins by mass. Western blot is an analytical technique to identify the presence of a specific protein within a complex mixture of proteins, where gel electrophoresis is usually used as the first step in procedure to separate the protein of interest. SDS-PAGE is by far the most common type of gel electrophoresis being used in western blot.