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 USAGE: 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 smartcard. /user <UserName> should be in form USER@DOMAIN or DOMAIN\USER program command line for EXE. See below for examples Examples: > runas /noprofile /user:mymachine\administrator cmd > runas /profile /env /user:mydomain\admin "mmc %windir%\system32\dsa.msc" > runas /env /user:email@example.com "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.