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 and keep them logged in. 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 logging out of accounts when finished, using secure connections (like HTTPS), and enabling two-factor authentication. Additionally, web developers can implement security measures like regenerating session IDs and setting expiration times for sessions to enhance protection against such attacks.