A donor eNB provides the radio connection for LTE (L1 or L2) relays, which extend the LTE air interface beyond where the donor itself can reach (for extended coverage, in-building penetration, etc.). The concept is similar to a repeater in other wireless technologies.
See the following image
In the above image MME1/SGW1 or MME2/SGW2 the visibility is DeNB but users are connected to DeNB via relay node not directly to DeNB. So without changing the core infrastructure by the use of RN and DeNB, operator is able to provide the service to the non-reachable area.
Now I believe its clear how DeNB is different from NodeB.