So, yes, you have to write a cover letter for your resume. Especially, if you need to explain employment gaps or if you're changing careers.

Does a resume contain a cover letter?

No, a CV does not include a cover letter. A curriculum vitae (CV) is a resume-like document that highlights your academic and professional experiences. ... A CV paired with a cover letter will give you the greatest chance of getting a job interview.

Should I attach my cover letter to my resume when they ask for resume?

While some experts recommend placing the cover letter first in a combined document, it's safer to start with the resume. The reason for that is that you want hiring managers to see the resume right away. Some may assume that it's just a cover letter if that's the first thing they see.