A Threat can be defined as anything which is danger to an organization's Asset. Threats can be physical threat of network based threats. An attacker may exploit poorly configured network infrastructure devices like a firewall, router network servers or switches. Default settings network infrastructure devices, loose access controls, applications and operating system without proper updates etc. are vulnerabilities and can be exploited by an attacker.
Countermeasure is any action to prevent a threat against a vulnerability. Countermeasures can be of following types.
Administrative Countermeasures: Security policies, general procedures, accepted safety guidelines etc can be considered as Administrative Countermeasures.
Physical Countermeasures: Physical security for Server Rooms, Network Infrastructure devices, Data centers, Accident and Fire Prevention, Uninterrupted Power Supply, Video Surveillance etc can be considered as Physical Countermeasures.
Logical Countermeasures: Proper configuration of network Firewalls, application and operating system password security, IPS (Intrusion Prevention Systems), VPN (Virtual Private Network), etc are examples of Logical Countermeasures .