AI is the study of how computation can produce intelligent behaviour, whether that is in a robot, a mobile phone, an internet search engine, or a computer game.
A knowledge engineer creates knowledge-based computer systems to solve complex problems normally requiring a high level of human expertise.
A machine learning engineer creates systems that utilise data from multiple sources to make predictions about future actions. i.e. the ability for a system to learn without explicit programming. Example systems include search engines and demand forecasting software to help manage aid distribution during a natural disaster.
A robotics engineer is responsible for creating robots and robotic systems to perform duties that humans are unable or choose not to complete.