Both a web app and a responsive web are designed to work on any device (smartphone, tablet, laptop..). They both are accessed through a web browser.
The main difference is that a responsive websites use different technologies to be created, and such technologies allow the programmer to not having to do any coding changes for the different devices, whereas for the web app those changes have to be made.
In terms of SEO, the responsive webs rank higher (they are also more expensive to build) and I would guess that’s because the content displays faster.
