A Next Gen Firewall (NGFW) or Unified Threat Management (UTM), are a part of third generation firewall technology appliance with consolidated functionalities of Firewall, Gateway Anti-Virus, Intrusion Detection and Prevention (IDP), Application filtering with Deep Packet Inspection (DPI), SSL encrypted traffic inspection, Data Loss Prevention (DLP) and third party identity management integration like Radius, AD, PMS etc.

Technically there are not any differences in a UTM & a NGFW and generally have same capabilities.
It was earlier believed that a UTM can not be fast and serve enterprise network. Hence the term NGFW was coined to create/market a product class capable of fitting into an enterprise network and include all features of UTM. But as the hardware has evolved and progressed, both the devices converged and now are interchangeably used by vendors/customers.

These devices have eased the management of network but disadvantage involved is Single Point of Failure (SPoF) within IT infrastructure.