Smart Card - A smart card is a credit card sized card that has an embedded microchip and one or more certificates. The information on the card identifies the user and includes the user’s private key used for asymmetric cryptography.
Users are often required to enter a personal identification number (PIN) along with the smart card. Using a smart card (something you have) and a PIN (something you know) provides multifactor authentication. Combining two or more factors of authentication is more secure than using only a single factor.
Both a CAC and PIV provide the same benefits of a smart card, but also include photo identification.
CAC - A common access card (CAC) is a smart card used by employees and other personnel in the United States Department of Defense (DoD). A CAC includes a picture of the user along with other information such as their name. DoD employees wear the CAC as a badge and can show it to guards to prove their identity. They can also use it as a smart card to log onto systems. PIV - A personal identity verification (PIV) card is also a specialized type of smart card used by personnel in United States federal agencies. Just as a CAC does, the PIV card includes a picture of the user along with their name. A PIV can be used for visual verification of users, and then as a smart card when users log onto their computer.