NTLDR is the file responsible for booting the operating system from storage devices like CD-ROM, USB etc. It also helps in booting non-Windows based Operating System. The NTLDR has two parts, first part it contains the boot loader itself and the second part contains BOOT.INI, which contains the configuration options for the boot menu. 
NTLDR need NTDETECT.COM to boot an NT based operating system.When booting the operating system the NTLDR performs the following actions:
It first places the processor in a flat 32-bit mode.
It then accesses the file system on the primary drive or the boot drive.
It reads the contents of BOOT.INI file and prompts the user with OS menu.
Incase an operating system other than windows exists then it reads the configuration from BOOT.INI and it loads the file called BOOTSECT.DOS and then passes on the control to it.
If it's a Windows NT based OS then NTLDR loads NTDETECT.COM, which collects information about the hardware in the computer.
NTLDR runs a file called NTOSKRNL.EXE with the information returned by NTDETECT.COM to it.
