NT Loader (NTLDR) is a boot loader for the Windows NT operating system. It is responsible for loading the operating system kernel and all essential device drivers during the boot process. NTLDR was first introduced in Windows NT 4.0 and continued to be used in Windows 2000, Windows XP, and Windows Server 2003.

During the boot process, NTLDR is loaded by the system BIOS from the boot sector of the system drive. The boot sector is the first sector of the hard drive and contains the master boot record (MBR) and the partition table. NTLDR then loads the operating system kernel and all essential device drivers into memory, and transfers control to the kernel.

NTLDR is designed to be extensible, allowing it to support additional features and capabilities through the use of boot sector plugins. These plugins can be used to add support for new file systems, hardware devices, and other features that are not natively supported by NTLDR.

NTLDR also provides a number of advanced features, such as support for multiple boot configurations and the ability to boot from non-standard media, such as USB flash drives. Additionally, NTLDR includes a number of diagnostic and recovery tools that can be used to troubleshoot and repair boot problems.

Despite its advanced features, NTLDR has been largely replaced by newer boot loaders in more recent versions of Windows. Windows Vista, Windows 7, and later versions of Windows use the Boot Manager (BOOTMGR) boot loader, which provides additional features and improved performance over NTLDR.

In conclusion, NTLDR is an important component of the Windows NT operating system, responsible for loading the operating system kernel and all essential device drivers during the boot process. While it has been largely replaced by newer boot loaders in more recent versions of Windows, it remains an important part of the history of the Windows operating system and an essential tool for anyone working with older versions of Windows.