It is preferable for letters of recommendation to be submitted via the online application system. (apply now.) All applicants applying for financial aid through the Graduate School must submit three letters of recommendation.
Letters of recommendation that are submitted offline (i.e., by mail, fax, hand-delivered) should be in a signed and sealed envelope from the author.
Letters should comment on the applicant's past academic record and potential for future success. Ideally, letters will be written by professors who have had you in class. In cases where that becomes impractical, letters from employers and character references can suffice.
Most programs do not require a specific recommendation form or format. A free-form letter is typically all that is required. There are some programs that require a recommendation form to be filled out instead of or in addition to a letter of recommendation. Check your program of interest's application requirements for details.