I am applying a position, which requires applicants to send cover letter and other materials by email. I think that in the application email I would present almost the same content in the cover letter: my interests in the job, my achievement, etc. So I am wondering that whether a cover letter as an attachment is still necessary or not. Any more information needed in the cover letter?
Write a traditional cover letter and attach it to your email. The committee reviewing your application may not get forwarded your original email. They will probably get a packet sent to them that's prepared by a staff member that aggregates all the submissions. Your email can be short and sweet: "Dear Whoever, Here are my application materials for the position Professor of Blah Blah Blah as advertised at URL. Please see attached for all the requested documentation."
A formal cover letter is necessary in some disciplines such as my own, where it serves as the short proxy for the research and teaching statement (i.e., you are expected in the space of 2 pages to summarize the significance of your research and if applying to a SLAC then the general focus of your teaching). There is great variance between disciplines though -- some require separate teaching and/or research statements. Ask your advisor what is appropriate for your field.
In any case, the e-mail containing the application itself can be short -- as the e-mail will often get deleted or otherwise not included in your file:
To: <email@example.com> Subject: Application for the junior position in sociocultural anthro (job #987) Dear Search Committee Please find enclosed my application for the junior faculty position in sociocultural anthropology at the University of the Bermudas (job posting #987). The attachments (enumerated...) are included in Adobe PDF format. Please let me know if any files are missing or cannot be opened. Warmly, Jane Smith <attachments including cover letter, cv, etc.>
I believe generally there isn't any big difference between a cover letter and an application email except the length. In the email you can simply state for which position you are applying and why you are applying but in a cover letter you give more information about yourself, your skills and achievements. Also, in the email you should mention why you are qualified for the position. Well, I guess the same should be mentioned in a cover letter. But as I mentioned above length is the difference so keep your mail short and to the point but write a compellingly interesting cover letter.