Drives are just like physical hard disks or CDs. They store your operating system, applications and data, and are persistent across server reboots.
Servers are virtual machine instances, and may have one or more drives attached to them. They are either running or powered off. When a server is running, the cloud provides VNC access. When a server is powered off, you can configure the CPU, memory, drives and IP addresses which it uses.
Some of our competitors have less-flexible products in which every server has exactly one drive. Our approach is more general, allowing you to attach several drives to a single server, or building several alternative configurations which boot from the same drive. (However, only one running server can access a given drive at any one time).