What is the difference between a volumetric DDoS attack and an application layer DDoS attack, and how do they relate?

Volumetric DDoS attacks flood the network with a large amount of traffic (bandwidth exhaustion), while application layer DDoS attacks target specific application vulnerabilities. While distinct, they are often combined. An application layer attack can be a component of a larger volumetric attack, or the attack can be both volumetric and application layer.