If the server is set to stay "24h/24h online" it will act as a normal gameserver: it will stay always on regardless if in use or idle.
The other possibility is "stop when empty".
It's environmental friendly as it allows better resources allocation shutting down unused gameservers when left empty for 30minutes.
It is very easy to use: in your control panel you will find a link you can give to anyone.
Whoever have that link can open it and force the server to start without being able to alter any other option (he does not needs to know your login details) :)
On the same section there's also a downloadable zip file that does the same thing: it allows anyone who got it to start your gameserver with just a click!
This is how the control panel appears (click to enlarge):