Whether you know if or not, you have a personal brand. This is a fairly new term, borrowed from marketing, that harkens to the old fashioned word, "reputation."
Your brand, from a work place point of view, identifies your level of expertise, who you are as a person, your work place value. Your self-marketing documents, resume and cover letter, should "sing" with your personal brand!
If you are an administrative assistant, I hope your resume's lyric conveys organization and time management along with appropriate software knowledge; if you are a help desk tech-type, I hope your resume plays a tune that conveys outstanding customer service, expert software knowledge, and great problem solving; if you are a doctor, I trust your resume is humming with proof of bedside manners, thorough medical knowledge, and the upholding of medical and corporate values!
What music is coming from your self-marketing documents?