What is the difference between signed and unsigned APK?

A signed APK contains a digital signature that confirms its integrity and the developer's identity. An unsigned APK does not have such a signature.

Characteristic	Signed APK	Unsigned APK
Integrity	Verified during installation/update	Not verified
Identification	Identifies the developer	Not identified
Trust	High (comes from a known source)	Low (origin unknown)
Installation	Possible on devices	Impossible on devices without special permissions
Update	Possible with the same signature	Impossible
Publication	Mandatory for Google Play Store	Impossible for Google Play Store