Session hijacking is a type of cyber attack where an attacker takes control of a user's active session on a website or application. This is often done by stealing session cookies, which are small pieces of data that identify a user during their online activities. Once the attacker has access to these cookies, they can impersonate the user and gain unauthorized access to their account. To prevent session hijacking, users should take precautions such as using secure connections (like HTTPS), logging out of accounts when finished, and avoiding public Wi-Fi for sensitive transactions. Additionally, websites can implement security measures like two-factor authentication and session timeouts to enhance protection against such attacks.