The term Human-computer interaction (HCI) is a research field, wider covering - you guessed it - how humans understand and interacts with computer systems. It is also an umbrella term many of the smaller design topics - such as UX and IxD - as well as aspects of fields like design, computer science, CSCW, behavioural sciences, linguistics, communications and more. It can be described as an academic field, rather than a more commercially applicable term, such as UI Design. This means it is generally used to articulate and explore the relationships between man and machine, and to a lesser extent imagining new ways of fostering that relationship.
The main reason why the emphasis is put on interaction with computers and not merely use of them is that computer systems often invites users to do many things with computers and that the relation between human and computer can thereby become interactive rather than a matter of utilization.