The most frequent types of authentication available in use for authenticating online users differ in the level of security provided by combining factors from the one or more of the three categories of factors for authentication:
# Single-factor authentication
As the weakest level of authentication, only a single component from one of the three categories of factors is used to authenticate an individual’s identity. The use of only one factor does not offer much protection from misuse or malicious intrusion. This type of authentication is not recommended for financial or personally relevant transactions that warrant a higher level of security.<ref name="Turner-DigitalAuthentication-Basics" />
# Two-factor authentication
When elements representing two factors are required for authentication, the term ''two-factor authentication'' is applied—e.g. a bankcard (something the user '''has''') and a PIN (something the user '''knows'''). Business networks may require users to provide a password (knowledge factor) and a pseudorandom number from a security token (ownership factor). Access to a very-high-security system might require a mantrap (mantrap (access control)) screening of height, weight, facial, and fingerprint checks (several inherence factor elements) plus a PIN and a day code (knowledge factor elements), but this is still a two-factor authentication.
# Multi-factor authentication
Instead of using two factors as used in 2FA, multiple authentication factors are used to enhance security of a transaction in comparison to the 2FA authentication process.
# Strong authentication
The U.S. (United States) government (United States government)'s National Information Assurance Glossary defines '''''strong authentication''''' as
layered authentication approach relying on two or more authenticators to establish the identity of an originator or receiver of information.
The Fast IDentity Online (FIDO) Alliance (FIDO Alliance) has been striving to establish technical specifications for strong authentication -infosecurity-magazine.com 
# Continuous Authentication
Conventional computer systems authenticate users only at the initial log-in session, which can be the cause of a critical security flaw. To resolve this problem, systems need continuous user authentication methods that continuously monitor and authenticate users based on some biometric trait(s). A study used behavioural biometrics based in writing styles as a continuous authentication method.