The Physical Extent (PE) are simple block containg data from the point of view of the physical device. You can compare them to blocks that compose a partition.
One or more PE compose a Physical Volume.
The Physical Volume is a device that can be used to store datas. It's normaly a HD or a partition, but can also be a software that emulates a HD (e.g.: software raid).
One or more PV compose a Volume Group.
The Logical Extent (LE) are simple block containg data from the point of view of the abstracted device.
Each LE is associated to a PE. Both have the same size but not all the PE must be on the same partition or disk!
One or more LE compose a Logical Volume.
The Logical Volume is equivalent to a partition in a not LVM system. It contains only one filesystem and this is that that will be mounted (e.g.: /home).
One or more LV compose a Volume Group.
The Volume Group is an abstraction that groups one or more LV and one or more PV.
Thanks this abstraction, the LV are independent from the PV and can therefore be split on more PV according your wishes.
With DISK I refere me to a disk device. This can be hda, hdb, ... for IDE devices and sda, sdb, ... for SCSI devices.
With PARTITION I refere me to a partition device. This can be hda1, hda2, ... for IDE devices and sda1, sda2, ... for SCSI devices.
With GROUP I refere me to a Volume Group name. This is a name that you can assign as you like.
With NAME I refere me to a Logical Volume name. This is a name that you can assign as you like.