It can be both.
Primarily it is the Java Virtural Machine (JVM) 'application server' on which the ColdFusion Java Application runs. JRun is not the only JVM that ColdFusion can run on, but it comes with ColdFusion if you don't have another one.
It also has a built in web server so it can full this role as well. This web server is recommended only for development or testing purposes since it is not as fully featured as other web servers and would be a poor fit for a production server.