Services, functions and routines

Some Windows internals terms have different meanings based on it's context.

Term	Description	Example
Windows API functions	Documented, callable subroutines in the Windows API.	CreateProcess
Native system services (or system calls)	Undocumented, underlying services that are callable from User-mode.	NtCreateUserProcess is the internal system service that the Windows CreateProcess function calls.
Kernel support functions (or routines)	Subroutines of the Windows OS called from Kernel-mode.	ExAllocatePoolWithTag, device drivers call this to allocate memory from the Windows system heaps (pools)
Windows services	Processes started by the Windows service control manager.	The Task Scheduler service runs in a User-mode process that supports the schtasks command.
Dynamic Link Libraries (DLLs)	Callable subroutines linked togheter as a binary file that can be dynamically loade by programs that use these subroutines.	Msvcrt.dll, Kernel32.dll, etc.