Natural Language Understanding (NLU) uses algorithms to covert data spoken or written by the user into a structured data model. It does not required in-depth understanding of the input but have to deal with much larger vocabulary.
Two subtopics of NLU:
Intent recognition: It is most important part of NLU, because it is the process of identifying user's intention in the input and to establish the correct meaning.
Entity recognition: It is the process of extracting most important data in the input such as name, location, places, numbers, date,etc. For example: What happened at 9/11 (date) in USA (place)