The cover letter is your chance to stand out from the other candidates by being persuasive and convincing in your arguments. It is worthwhile, therefore, to spend some time thinking about how best to present yourself so that the employer will be more likely to take your CV/application seriously. Address by name if you can e.g. "Dear Ms Fletcher and" conclude with "Yours sincerely". If you have no name, place a telephone call to the organisation, find out the Recruitment Manager's name and address your cover letter/email to them.
What your Cover Letter should include
Identify who you are and explain why you are writing (saw advertisement online or in paper, speculatively,: ), and mention the fact that your CV is enclosed.
Continue by detailing why you are interested in the job and the employer, your relevant project work and work experience and what skills and qualities you will bring with you to the company.
Outline your current situation.
Say why they should be interested in you and emphasise your main 'selling points'.
Conclude on a polite and optimistic note, saying that you look forward to their response and that you are available for an interview at their convenience.
Ensure you use good grammar and proper spellings - don't rely on a spellchecker and don't use americanised spellings when applying to European companies (e.g. use realise, not realize; use programme, not program!).
The letter should be an appropriate length. One side of one A4 is usually sufficient.
Do not simply repeat the information that is already on your CV. Highlight and expand on the relevant items.
When making application to specific companies, check if they have a careers section on their website - many will provide their own advice on CVs, such as Bank of Ireland. You should, of course, thoroughly research a particular company's website upon making an application to them.