Disks come into two variants. Unmanaged disks are page blobs in storage accounts, manually managed independently of the VM. Managed disks are the recommended approach, allowing you to set and forget storage without having to consider storage account IOPS limits, managing access or replication topologies. It's possible to convert unmanaged disks to managed disks. We pay for pre-allocated storage.
Snapshots are full, read-only copies of a managed disk.
Images are generalised VM disk images to be used as base images, and can be created from snapshots.