What is the difference between a functional and a non-functional requirement?
Functional requirements describe what the system does: behaviours, features and business rules. Examples include calculating a discount, generating an invoice or allowing a user to reset a password.
Non functional requirements describe how well it does it: quality attributes and constraints. Examples include performance, security, availability, scalability, usability, compliance and accessibility.
Both matter, but non functional requirements are often skipped and then cause expensive problems later. A feature that works but takes ten seconds to load, fails under load, or is inaccessible is not acceptable.
They should be specific and measurable, or they cannot be tested. Instead of "the system should be fast", write "the search results page must load in under two seconds for ninety-five percent of requests at peak load".
I capture them early, include them in acceptance criteria and trace them to tests so they are verified, not assumed.