NTLM (NT LAN Manager) is a suite of Microsoft security protocols used for authentication, session security, and message integrity. 

NTLM is commonly used for authenticating users in Windows domain environments. It supports both local and network authentication, allowing users to log in to their local machines as well as access resources on a network. NTLM uses a challenge-response mechanism where the server sends a challenge to the client, which the client must encrypt using its password hash before sending it back to the server for verification.