A motivational letter or cover letter
But what's the difference?
A motivational letter is a one-page letter that’s used to describe why you are the perfect candidate for a certain position. It is usually attached to your resume.
The motivation letter which is not only required for work, it can also be for university admission or when applying for a scholarship. If you are applying for a job, you should rather state why you are there and what motivates you to work there, what your competencies are.