Here's a topic that's being discussed often - the lines of division can be blurred sometimes (due to marketing and other factors), hence I'll try to clarify this.

https://www.slideshare.net/andrewplato/utm-vs-ngfw-a-single-shade-of-gray

UTM - the term was coined around year 2004 (I believe it was coined by IDC Research), basically it describes a product which combines multiple security features 

NGFW - this term showed up around 2010 / 2011, I believe it was coined by Gartner and / or Palo Alto Networks, boasting application control (hence the Next-Gen prefix)

UTM = FW + IPS + IDS + Gateway AV

NGFW = FW + QoS (for DPI) + IPS + (SSL / SSH inspection)