Steven Rothberg, President and Founder of collegerecruiter.com, web site dedicated to providing jobs, information and advice for college graduates and entry-level job seekers, provided these thoughts:
Rothberg: Congratulations on receiving your job offer letter, especially if you've worked hard to receive the offer and want to work for the employer. You are absolutely correct to want to respond appropriately to this written offer of employment. Perhaps you already realize it but how you respond to the letter may be your first, formal, written communication with your employer and could have a significant positive or negative impact on your relationship with that employer.
Before you put pen to paper or, more realistically, fingertips to keyboard, carefully read through the job offer letter. It may provide specific instructions or even a form for you to complete. It may not even request a response but instead be designed simply to communicate to you when you should show up for your first day. I suspect that your letter falls into the third bucket: it was a formal, written offer of employment and therefore requires a formal, written acceptance of that employment.
The offer letter should include the terms of employment such as your job title, responsibilities, compensation, benefits. If not, call or email your hiring manager or a human resources representative to ask for that detail. They can hardly expect you to accept their offer if you don't know what they're offering, right?
Once you have all of the employer's proposed terms, call or email your hiring manager or human resources representative to ask any questions you may have. You may have none but if the compensation or some other term of employment isn't what you want, this is the time to negotiate. If you're going to ask for a change to a term, justify your request. For example, let's say the offer is for a salary of $40,000 per year but your research indicates that similarly qualified people in similar positions with similar organizations in similar locations are paid $50,000 per year. Provide that research to your hiring manager or human resources representative in a phone call or email and ask them to match the going rate. Most organizations want to pay their employees fairly. If they failed to do the necessary research then they'll probably be grateful to you for doing your homework and will respect you even more than they already do. If they did the research and disagree with your analysis then you have the opportunity to discuss the situation with them and work together to determine whose analysis is correct or if the answer lies somewhere in between
You should now have resolved any disagreements and essentially have a handshake agreement. Now write a memo or letter accepting the offer. If they wrote a formal letter to you and mailed it, then you should write a formal letter to them and mail it. If they emailed an informal note to you, then you should email an informal note to them. Regardless of how you accept their offer, but sure that your acceptance is simple and concisely re-states their offer in the same manner as they presented it to you so if their letter used bullet points, so should yours.