Httpd
httpd is the web service. When you create a web application this is the service that is used, along with all of the other web based applications on Amahi.
If for some reason it does not start, or when you try to manually start it "sudo service httpd restart" and you get a return of [failed] first thing you should do is check the error log. This is located in "/var/log/httpd/" you will need to be Super User to access these logs.
The best way to view the error log is to open terminal and type "sudo nano /var/log/httpd/error_log" Look at the bottom of the file and it should provide you with the most recent error.
This will help you troubleshoot why the websites on your HDA are not showing up.
Note: if you delete the log files for a custom made web application this will cause an error that will prevent httpd from starting. Find out what directory it is looking for and recreate that manually. Once this happens you will be able to start the httpd service.