The RunAs service allows you to run an application as different user. You typically launch applications in this mode via the command line or shortcut. The "Run As" service does not allow you to run your application as a service - it is a built in Windows service (just to add to the confusion!).

FireDaemon on the other hand allows your application to run as a service and optionally as a different user without the need for the Secondary Login / Run As service to be enable.

For more information about the RunAs (Run As) service please see this Microsoft KB article:

Here's the command line syntax for the RunAs.exe command line utility:

C:\>runas /?

RUNAS [ [/noprofile | /profile] [/env] [/savecred | /netonly] ]
        /user:<UserName> program

RUNAS [ [/noprofile | /profile] [/env] [/savecred] ]
        /smartcard [/user:<UserName>] program

   /noprofile        specifies that the user's profile should not be loaded.
                     This causes the application to load more quickly, but
                     can cause some applications to malfunction.
   /profile          specifies that the user's profile should be loaded.
                     This is the default.
   /env              to use current environment instead of user's.
   /netonly          use if the credentials specified are for remote
                     access only.
   /savecred         to use credentials previously saved by the user.
                     This option is not available on Windows XP Home Edition
                     and will be ignored.
   /smartcard        use if the credentials are to be supplied from a
   /user             <UserName> should be in form USER@DOMAIN or DOMAIN\USER
   program         command line for EXE.  See below for examples

> runas /noprofile /user:mymachine\administrator cmd
> runas /profile /env /user:mydomain\admin "mmc %windir%\system32\dsa.msc"
> runas /env / "notepad \"my file.txt\""

NOTE:  Enter user's password only when prompted.
NOTE:  USER@DOMAIN is not compatible with /netonly.
NOTE:  /profile is not compatible with /netonly.
NOTE:  /savecred is not compatible with /smartcard.