Functional requirements are specifications of the functions or features that a system or its component must have to enable users to accomplish their tasks. They define the basic behavior of the system under specific conditions. Functional requirements are different from non-functional requirements, which describe the general properties of a system, such as performance, security, or reliability.

Functional requirements are expressed in the form "system must do <requirement>," and they drive the application architecture of a system. They are supported by non-functional requirements, which impose constraints on the design or implementation. Functional requirements are detailed in the system design, whereas non-functional requirements are detailed in the system architecture.

Functional requirements are essential for software development projects as they provide a clear description of how the system is supposed to respond to a particular command, the features, and what the users expect. They are crafted by business analysts who communicate with clients, figure out their needs and wants, and translate them into specifications.

Examples of functional requirements include:

• The system must send a confirmation email whenever an order is placed.
• The system must allow blog visitors to sign up for the newsletter by leaving their email.
• The system must allow users to verify their accounts using their phone number.

Functional requirements are necessary for the success of any software development project as they ensure that everyone on the team understands what needs to be built and can prioritize their work accordingly.