ContainerWare is an application server, similar in concept to Tomcat or Jetty, except that it's written in C and the applications (containers) are implemented as dynamically-loadable modules.
It listens on one or more endpoints (which might be TCP/IP ports, FastCGI sockets, etc.) and passes requests to application instances for processing. Depending upon the engine processing the request, the instance might be one of a pool of threads, or child processes, or something else.