书信(Letter)一般可分为事务信件或公函(Business Letter or Offical Correspondence)以及私人信件(Private Letter)两大类。
1.写作要求：
一般包括以下几个要点：
1) 收信人的姓名和地址;
2) 寄信人的姓名及其与收信人的关系;
3) 寄信人的地址(有时可以省略);
4) 写信的时间;
5) 信的.内容。
1. 书信的格式
1) 信头(Heading);
2) 称呼(Salutation);
3) 正文(Body of the letter);
4) 结束语(Complimentary close);
5) 签名(signature)。
2. 书信的种类
邀请信有正式和非正式之分，也有介于两者之间者。正式邀请信一般属于公函类，私人邀请信一般属于非正式文体。邀请朋友、熟人参加某一聚会时，再词句上不一定多加斟酌，用词太正规倒显得关系不密切。但是如果与被邀请人不太熟悉的话，最好用正式文体。如虽认识但并不十分了解，文体可介于两个者之间。由此可见，私人邀请究竟用什么文体，关键看与被邀请人之间的亲疏的程度。邀请信要明确写出活动(是晚会、晚餐还是一般的聚会等)、活动的时间地点组织这项活动的理由，有时还应告诉对方可能出席的人，另外一般希望对方给予答复。
(1) 非正式邀请信，如;
26 Coventry St.
London
Sept. 15th, 1999.
Dear Jack,
I am going to the cinema to see the Red River Valley and I have two tickets. Would you like to come? The film starts at 7:30 p.m. Maybe we can meet at the Covent Garden Station at 7:00 p.m. and have a drink before the film starts. Please phone me at home to let me know.
Looking forward to seeing you.
Love
Mary
这封信是写给朋友的，所以用语比较随便，比较口语化。
(2) 正式邀请信，如：
Dear Ms. Gupta;
It’s our great pleasure to announce that Jan. 19th is the 3rd anniversary of the founding of our corporation. We have a small party as follows: 7:00p.m.,Jan. 19th, Room No.123, Kunlun Hotel. At this moment, we would like to express our sincere gratitude to our clients for both courtesy and support over the past three years. Please confirm if you plant to attend.
Yours Sincerely,
Lewis
这是一封正式邀请信，十分正规，无论从句式，还是用语都十分讲究。
【参考范文】：
Dear Xiao Hong,
I am really glad to receive your letter. In your letter you asked me to give you some advice on how to improve your writing ability. It is not easy to give such advice. But in my opinion, you can only improve it by writing more. The more you write, the better you can write. I suggest that you keep diary every day. You can write down what you see and hear in the day in your diary. It's a good way to practice writing. Just have a try.
I am not sure whether my advice will be of any help to you. But don't hesitate to write to me if you have any more questions.
Best wishes.
初中英语作文写作指导：书信的格式
2.书信写作格式