User Stories

“As a [type of user], I want [some goal] so that [some reason].”

As a [role], I can [feature] so that [reason].

As a [role], I want [feature] because [reason].

As a [role], I can [feature].

Advantages of this approach. Don't forget to make use of Acceptance Stories as well.

The Testing Geek has a good explanation of User Stories.