Two-factor authentication (2FA) is a security mechanism designed to enhance the protection of online accounts and systems by requiring users to provide two types of authentication to verify their identity. These authentication factors fall into three main categories: something the user knows, something the user has, and something the user is. 

The first factor is typically something the user knows, like a password or a personal identification number (PIN). The user usually has the second factor, such as a mobile device, security token, or smart card. Additionally, biometric data like fingerprints or facial recognition can serve as the second factor, representing something the user is. 

Combining two factors significantly strengthens security compared to relying on a single factor. Even if one factor is compromised (e.g., a stolen password), the attacker would still need the second factor to gain access. This added layer of protection helps prevent unauthorized access and mitigate the risks associated with credential theft and phishing attacks. 

Two-factor authentication is widely used across various online platforms, including email services, banking systems, social media platforms, and more. It provides a critical defense against unauthorized access to sensitive data, personal information, and financial accounts. 

Understanding Two-Factor Authentication (2FA) is essential for users to adopt secure practices when accessing online services. It's also crucial for businesses and organizations to implement 2FA as part of their security measures to safeguard user data and maintain a high level of trust with their user base.