MDF (Main Distribution Frame): central wiring hub, core/distribution switches, main internet connection.
IDF (Intermediate Distribution Frame): branch wiring closets, access layer switches serving floor or wing.
MDF-to-IDF connections typically use fiber (higher speed, longer distances). IDF-to-desktop uses UTP copper. Physical security of MDF/IDF is critical — unrestricted physical access to switches = full network access. Lock wiring closets, install cameras, control who has keys.