While both types of developers write software to create user-facing applications, web developers build products on web technologies that run in a browser (e.g., Google Chrome, Internet Explorer or Mozilla Firefox), while app developers use a variety of (often platform-specific) languages to create a more consistent and natural experience within the operating system.
Related Careers

Psychologist
A psychologist is someone who studies mental processes and human behaviour by observing, interpreting, and recording how people relate to one another and the environment.
Social Worker
There are two main types of social workers: direct-service social workers, who help people solve and cope with problems in their everyday lives, and clinical social workers, who diagnose and treat mental, behavioural, and emotional issues.