The document discusses defects in software testing. It defines a defect as an error or bug in an application that causes it to not meet user expectations or software requirements. Tests can show the presence, not absence, of defects. There are various types of defects, including bugs, failures, mistakes, and errors from different perspectives. Defects are categorized as functional or non-functional. Examples are provided for different types of defects like wrong, missing, and extra. The document notes that finding and fixing defects later in the software development process costs significantly more than fixing them earlier.