Partitions are storage units used to divide physical disks into smaller, independent parts. Basically they're containers for filesystems. Volumes are storage units with a filesystem. A volume can be inside a partition, but it's also possible for a volume to span multiple partitions.
As for assigning a drive letter to that particular partition: you need to determine the type of the filesystem inside that partition first. GParted might prove helpful there. Windows can only mount FAT(32) and NTFS volumes. For other filesystems (like Ext2/3/4 or XFS) you're going to need 3rd party tools.