What should be included in a cover letter?

You may be required to submit a cover letter with your submission. Individual journals may have specific requirements regarding the cover letter's contents, so please consult the individual journal's Guide for Authors.

A cover letter is a simple, brief business letter, designed to introduce your manuscript to a prospective Editor.  If the Guide for Authors does not specify what to include in your cover letter, you may wish to include some of the following items:

• Specify special considerations that should be given to the paper (if any).
• A brief background regarding the research involved or how the data was collected.
• Details of any previous or concurrent submissions.
• It's also useful to provide the Editor-in-Chief with any information that will support your submission (e.g. original or confirmatory data, relevance, topicality).
• The inclusion (or exclusion) of certain Reviewers (if propose/oppose reviewers isn't an available step in the submission process).
• Bring to the Editor’s attention any Conflict of Interest or Permissions information which may be relevant.  Be sure to upload any accompanying forms or declarations as required to your submission.

via Elsevier.