] a cover letter is a brief introductory letter(email) to a recruiter/company/? outlining/highlighting your "qualifications" for a particular "position"
[WHY]
] to introduce yourself
]
] a max 1 page "reason" to get them(HR) to read your resume
[WHERE]
] usually a cover letter is included with your resume in an application for a position
] to hilite your features that match their requirements
[WHEN]
] applying for a position that exists or is being advertised