Humans and Computers
Human Computer Interaction (HCI) is the study of how people interact with computers and how successfully computers interact with human beings.
This relates to both the design of hardware and software.
Programs need to be intuitive and user-friendly. No matter how good your programming is, if people can't work out how to use it or it is difficult to use, it is unlikely to be successful.
Hardware needs to work in practice, while weight is not a hugely important factor for a desktop computer it is for a laptop and even more so for a phone. A phone that is too big is hard to carry around but one that is too small will be very difficult to use.
This area also encompasses ergonomic design and aesthetics.
Think of an example of technology you found easy to use, what made it easy? Now think of one that was difficult to use. What made it hard?