Cover letters: guidelines and samples

Why is a job search letter called a "cover letter?" Because in the era of hard copy resumes and letters, the letter would be on top of, and therefore cover, the resume. A cover letter introduces a resume and explains why you are sending the resume.

Writing guidelines

Do I have to include a cover letter with my resume?

If you're sending your resume via email, yes.
If you are speaking to an employer at a career fair (or other event), no.
If you are submitting your resume online (sometimes to an employer's Applicant Tracking System, called ATS; a thing that's good to know about), do what the employer instructs! If there are no instruction, a cover letter can help. A cover letter is personalized and individualized to the employer. (Your resume is not.)

In person, you have a conversation, so you're talking to the employer about why you're interested in their job, and offering your resume. A letter doesn't apply to that situation. You can and likely should send a follow up email afterward to emphasize your interest and thank the employer for their time in talking with you.

When emailing your resume, your email usually serves as a cover letter; it explains why you're sending your resume. (In certain cases, if the employer requests this, or you are in a very formal and/or traditional industry, you might attach a cover letter document to your email.)

What a well-written cover letter (usually email) does:

What if I'm sending my resume to someone who knows me and is going to pass it on to someone else?

Email or Word docx / hard copy?

In most situations, email is the medium for professional communication in the job search.

Sometimes, in very formal and/or traditional fields, letters are written as documents with certain formatting. That document could then be sent as an attachment to an email (with a clear, concise description of what is attached and why). It's now very rare to actually physically mail hard copy when applying for jobs.