Written communication, whether hard copy or email, serves many purposes in a job search. In a thorough job search, you will write many types of letters.  Cover letters, which accompany and introduce your resume, are the ones you may hear about most, but are not the only letters you will need.  Letters also precede, follow-up and confirm verbal conversations, so they serve as a record and reminder of interactions, as well as evidence of your communication skills.

Basics for all correspondence:

Individualize
There are similar elements to job search letters, but each letter should be individually tailored and targeted to the recipient. There is no such thing as an effective "form letter" in a job search. You know when you get a form letter; a prospective employer knows too. A letter that looks like it could have been randomly sent to any employer is a good candidate for the employer's "no" pile.

Make your purpose clear
Don't make an employer guess why you are writing or what you are writing about. In choosing your words, think about the purpose of your letter and details of your individual circumstances. For example, if you make a telephone call to an employer prior to sending a cover letter, it makes sense for your letter to refer to the telephone call. If you must respond to an employer's letter to you, read the letter carefully to draft an appropriate response.

Tell the employer what you'll do for the organization, not what the organization can do for you.
Saying, "I really want this job because it will give me great experience," is not a sell to an employer. Of course the job will give you (or someone else) great experience. It just makes you sound "all-about-me." Instead, tell the employer what you have to offer. Be specific and realistic; as in, "I have great organizational skills that I developed and demonstrated when I was event chair for my club." Don't use hyperbole; as in, "I will immediately contribute to higher profitability on my first day on the job."

Edit
Grammar, spelling and punctuation should be error-free; wording should be clear, concise and business-like; avoid gimmicky language and slang terms.

Don't rely on spell check alone
Spell check won't let you know that you've used manger instead of manager, perspective instead of prospective, left the "l" out of public, and so on. (All mistakes we've seen plenty of times.)

Be yourself
Be your formal, business-like self, but express yourself in a manner that is natural to you. Avoid too much borrowing of language from sample letters and friends' letters. Excessively flowery language or using complicated words won't make you sound smarter; it will make you sound silly. Use good examples as inspiration, but don't copy.

Record-keeping
Retain copies of every letter you send and receive, including email; mark your calendar for any appropriate follow-up.