cover letter 是什么东西??和business letter 是一个东西嘛??

不是一个东西。
cover letter 的英文解释是 a letter sent along with other documents to provide additional information。就是随你的信一起发出，但是作为首页，提供你个人的一些信息及这个信件的概括说明什么的。
不知道你们老师要你们写什么类型的cover letter，一般来说，cover letter用在求职的简历、商业信函、或者其他文件。
我这里举一个求职的简历 cover letter 的例子。

Your Street Address
City, State Zip Code
Telephone Number
Email Address
（上面是你的资料）

Month, Day, Year

Mr./Ms./Dr. FirstName LastName
Title
Name of Organization
Street or P. O. Box Address
City, State Zip Code
（收信方的资料）

Dear Mr./Ms./Dr. （对方的姓）:

Opening paragraph: State why you are writing; how you learned of the organization or position, and basic information about yourself.
（第一段说明你写这个信的目的、你怎样了解到这个机构以及个人简介）

2nd paragraph: Tell why you are interested in the employer or type of work the employer does (Simply stating that you are interested does not tell why, and can sound like a form letter). Demonstrate that you know enou