Glossary
Overlay (appended data)
Data stored in a PE file after the end of its last section, which the Windows loader does not map: installer payloads, archives, configuration, signatures.
The overlay starts where the last section's raw data ends. Windows ignores it when loading the program, so it is a convenient place to append data: installers (NSIS, Inno Setup) and self-extracting archives keep their payload there, and droppers keep theirs. The Authenticode certificate table is also normally located in the overlay.
High entropy, a recognisable format (ZIP, 7-Zip, another PE) or data after the certificate table are worth a look. Data between the sections and the certificate table is covered by the signature; data after it is not.