Regarding the last question, LVM is a method for managing disks where you can assign physical volumes to volume groups (a group of physical disks) and then split the volume group into partitions as if it were a single disk, each division is called a logical volume.
You can easily resize a LV on the fly (e.g. if you run out of space), attach more disks to the VG, create new partitions, etc.
To the eyes of the kernel there is no difference between a regular partition and a Logical Volume
Quote:
VM is a method of allocating hard drive space into logical volumes that can be easily resized instead of partitions.
With LVM, a hard drive or set of hard drives is allocated to one or more physical volumes. A physical volume cannot span over more than one drive.
The physical volumes are combined into logical volume groups, with the exception of the /boot/ partition. The /boot/ partition cannot be on a logical volume group because the boot loader cannot read it. If the root (/) partition is on a logical volume, create a separate /boot/ partition which is not a part of a volume group.
Since a physical volume cannot span over multiple drives, to span over more than one drive, create one or more physical volumes per drive.
Linux LVM