Letters of introduction are mainly used to express interest in a job that has not been advertised, while cover letters are used to express interest in a job that has been advertised. A letter of introduction is the first contact with the employer, so it must make a lasting impression by being well written, well researched and original.
A letter of introduction must show the employer that the job seeker has something special to offer the company. The job seeker must, therefore, stand out.
A letter of introduction is frequently interchangeable with a cover letter. Many schools, for instance, treat them this way. However, strictly speaking, they are not the same.