[1] In the ‘real world’ passports and ID cards identify people; crests or symbols identify institutions like the police or a hospital; and a seal or stamp authenticates a document [2]. In the Digital World, we use Digital Certificates to do the same thing.
Digital certificates can be used for improved security where a username and password are used to login to any web application by introducing two factor authentication [3]; they can also be used to secure the connection to any server using SSL certificates [4]; and can be used for identity cards [5] and even national ID cards.
Digital signatures can be used for sign PDF [6] documents; a digital signature can be installed on a server for signing e-invoice [6], statement and other billing documents; and can also be used to secure the connection to authenticate legal transactions that occur over the internet.
Links:
[1] https://www.digi-sign.com/downloads/download.php?id=digi-ca-pdf
[2] https://www.digi-sign.com/digital+document
[3] https://www.digi-sign.com/digi-access
[4] https://www.digi-sign.com/ssl
[5] https://www.digi-sign.com/digi-card
[6] https://www.digi-sign.com/digi-seal