Logical disk

A logical disk is a concept from computing. Logical disks provide a certain amount of space to store data. Unlike real (called "physical") hard disks that can be touched, logical disks can take this storage space from different locations.
Common locations where this can be taken from:
The disk is described as "logical" because it does not actually exist as a single physical entity in its own right. 
Why do we need them?
When IBM first released the magnetic disk drive in the 1956 (IBM 305) a single drive would be directly attached to the using system, with each disk managed as a single entity. As the development of drives continued, it became apparent that reliability was a problem and systems using RAID technology evolved. This means that more than one physical disks are "RAID-ed" together to produce a single logical disk. 
In a modern home PC environment, disk drives now provide hundreds of gigabytes of storage capacity which can be impractical to use as a single entity. Therefore most systems have their drives partitioned into multiple "logical drives".
In most modern business IT environments, some form of a Storage area network will exist. Here, many storage devices are connected to many host server devices in a network. A single RAID array may provide some capacity to one server, and some capacity to another. Therefore logical disks are used to partition the available capacity and provide the amount of storage needed by each host from a common pool of logical disks.