Not sure what you googled Rob but when I use 'multiple domains tomcat' I get a heap of stuff and most pretty much lay it out.
Anyway, here is a shortcut.
In your server.xml file you can see the default localhost Host...
<Host name="localhost" appBase="webapps" unpackWARs="true" autoDeploy="true">
<Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
prefix="localhost_access_log" suffix=".txt"
pattern="%h %l %u %t "%r" %s %b" />
</Host>
Now you want you web site landing page for robapps.com
Create a folder under Tomcat/webapps called robapps and put your entire website in there and you index.html file also.
Now back in server.xml after the <Host></Host> tag above.... add this....
<Host name="www.robapps.com" appBase="webapps" unpackWARs="true" autoDeploy="true">
<Alias>robapps.com</Alias>
<Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
prefix="myfirstapp_access_log." suffix=".txt"
pattern="%h %l %u %t "%r" %s %b"/>
<Context path="" docBase="robapps" debug="0" reloadable="true"/>
</Host>
Save it and restart Aware. Make sure Tomcat restarts.
It will take a few minutes to work thru everything but you should be able to hit the site with your domain name http://www.robapps.com and you landing
page from Tomcat/webapps/robapps/index.html should load
Hope that works for you.