According to the Dictionary of Occupational Titles; an editorial assistant:

Prepares written material for publication, performing any combination of following duties: Reads copy to detect errors in spelling, punctuation, and syntax. Verifies facts, dates, and statistics, using standard reference sources. Rewrites or modifies copy to conform to publication's style and editorial policy and marks copy for typesetter, using standard symbols to indicate how type should be set. Reads galley and page proofs to detect errors and indicates corrections, using standard proofreading symbols. May confer with authors regarding changes made to manuscript. May select and crop photographs and illustrative materials to conform to space and subject matter requirements. May prepare page layouts to position and space articles and illustrations. May write or rewrite headlines, captions, columns, articles, and stories according to publication requirements. May initiate or reply to correspondence regarding material published or being considered for publication. May read and evaluate submitted manuscripts and be designated Manuscript Reader (print. & pub.). May be designated according to type of publication worked on as Copy Reader (print. & pub.) when working on newspaper; Copy Reader, Book (print. & pub.) when working on books.

Similarly, the O*NET describes the role of an editor as follows:

Prepare, rewrite and edit copy to improve readability, or supervise others who do this work. Read copy or proof to detect and correct errors in spelling, punctuation, and syntax. Allocate print space for story text, photos, and illustrations according to space parameters and copy significance, using knowledge of layout principles. Plan the contents of publications according to the publication's style, editorial policy, and publishing requirements. Verify facts, dates, and statistics, using standard reference sources. Review and approve proofs submitted by composing room prior to publication production. Develop story or content ideas, considering reader or audience appeal.
Oversee publication production, including artwork, layout, computer typesetting, and printing, ensuring adherence to deadlines and budget requirements. Confer with management and editorial staff members regarding placement and emphasis of developing news stories. Assign topics, events and stories to individual writers or reporters for coverage

For even more information, read more according to the Occupational Outlook Handbook, visit:

Proofreader / Editorial Assistant