"Volume" in Windows roughly corresponds to a filesystem instance. (It is not necessarily NTFS.)
(Although sometimes programs ask you to specify a volume when they actually use its underlying block device, since that's the easiest way to do it – while Windows does have an equivalent of /dev/sd? names, they're more obscure.)
The term doesn't imply any particular type of underlying block device; a volume might be stored on a traditional MBR partition, on a GPT partition, on a LDM partition, on whole disk, on Storage Spaces, etc.
A "drive letter" is one way to refer to a specific volume (like a mount point on Linux).
However, a volume doesn't necessarily have exactly one drive letter assigned – it might be mounted on a folder (Unix-style), or nowhere at all, or have both a drive letter and a folder mount, or have multiple drive letters.
(For example, the EFI system partition is not mounted on a drive letter by default, though it remains accessible through the special \\.\Volume{uuid} name.)
Additionally, a drive letter doesn't always refer to a volume root. On the NT side it's just an object symlink, and it can point to various places – most commonly regular folders (subst) or UNC paths (net use), which actually have been possible even back in MS-DOS.
