Block-level storage mimics the type of storage that would have originally been provided by hard disks and, later, storage arrays. Typically, block storage is exported via storage arrays via fiber channel or iSCSI onto hosts where a local filesystem is then formatted onto the block device. In some cases, this filesytem may be of the clustered type, and can allow the block device to be presented across many hosts at the same time. It's important to note that even though block-based storage allows you to present it to multiple hosts, this should only be done if the filesystem supports it; otherwise, corruption of the filesystem is highly likely.
One use of block storage that has seen a massive expansion in recent years has been through the use of virtualization. Block storage is quite often presented to a hypervisor that's formatted with a filesystem. One or more...