In a Windows network, NT (New Technology) LAN Manager (NTLM) is a suite of Microsoft security protocols intended to provide authentication, integrity, and confidentiality to users. NTLM is the successor to the authentication protocol in Microsoft LAN Manager (LANMAN), an older Microsoft product.
The NTLM protocol suite is implemented in a Security Support Provider, which combines the LAN Manager authentication protocol, NTLMv1, NTLMv2 and NTLM2 Session protocols in a single package.
Whether these protocols are used or can be used on a system is governed by Group Policy settings, for which different versions of Windows have different default settings. NTLM passwords are considered weak because they can be brute-forced very easily with modern hardware.