Definition of "Application Server"

One of the issues I've had using phpStorm is that the help seems to talk about something but not define what that something is. Maybe I'm just a bit dense, but could someone offer me a concise definition of "Application Server"? What differentiates this server from other types of servers, etc.? I can intuit, but would love for someone to s-p-e-l-l it out for me!

Thanks,

Dave