Software with a user interface displayed in a web browser. These applications may run on third-party servers or be self-hosted.
Web-Apps usually don't require you to install anything on your own device(s) other than a web browser – though sometimes you can do so (e.g. hosting them on your own server). These applications are installed on a web server, where they can be used from anywhere – on the condition that an Internet connection is available. Thus, in the ideal case, you have no maintenance tasks to perform (software updates are performed server-side).
Please do not confuse this with hosting questions. Your question should always be for the software – not for the place where it is installed (see: Are hosting recommendation requests on-topic?).