The "init.d" system is a part of the Unix and Linux operating systems that manages the starting and stopping of services during the boot process. It contains scripts that control various system services, such as web servers and databases, ensuring they run when the system starts and stop when it shuts down. These scripts are typically located in the "/etc/init.d/" directory and can be executed manually or automatically by the system. The use of "init.d" allows for better organization and management of services, making it easier for system administrators to maintain the overall health of the system.