In the ESXi context, the term target identifies a single storage unit that a host can access. The terms device and LUN describe a logical volume that represents storage space on a target.

The terms device and LUN mean a SCSI volume presented to the host from a storage target.

Different storage vendors present their storage systems to ESXi hosts in different ways. Some vendors present a single target with multiple LUNs on it. Other vendors, especially iSCSI vendors, present multiple targets with one LUN each.

Figure 1. Target and LUN Representations
There are two storrage arrays. The first one has one target with three LUNs and the second has three targets with one LUN each.

In Target and LUN Representations, three LUNs are available in each configuration. On the left, the host sees one target, but that target has three LUNs that can be used. Each LUN represents an individual storage volume. On the right, the host sees three different targets, each having one LUN.