A Cover Letter is usually used to communicate the main purpose of your application and gives you the opportunity to highlight particular areas of expertise, which may not be fully explained in your resume.

It is often helpful to the employer if you refer to how or where you heard about the vacancy for which you are applying.

Whilst your resume is vital for detailed information, the cover letter can be a real opportunity to sell yourself in the first instance, so should be well written, well presented and carefully checked for errors prior to sending.