The Corresponding Secretary is the official voice of the organization. If the President is the soul, the Second Vice President is the brain, and the Sergeant at Arms is the hands of the organization, the Corresponding Secretary is its gaping, toothy maw. The Bylaws describe the CorSec by saying:
The Corresponding Secretary shall have charge of all correspondence pertinent to the organization other than the correspondence reserved for the Recording Secretary and Treasurer. The Corresponding Secretary shall be the Editor of the organization Newsletter.
That is to say, the CorSec makes all official announcements, conducts all official correspondence on behalf of the organization (including reading and answering the email), reserves space for organizational meetings and events, and picks up the physical mail at the mailbox in the UC. In addition, the CorSec is the editor of Pravda?, that most exemplary of publications.
At times, the CorSec has also announced upcoming events, though this task is often best conducted by the Second Vice President. Also, the CorSec has historically been in charge of updating the website, though that task is large enough that an ad-hoc position would probably be appropriate.
The Recording Secretary used to be ranked fourth in seniority with Corresponding Secretary ranking just below at fifth. This ranking could theoretically be problemtaic; if the three highest ranking officers (President, First Vice President, and Second Vice President) were absent, but the four lowest ranking officers (both secretaries, Treasurer, and Sargeant at Arms) were in attendance, then the meeting would still proceed because officer quorum would be met. Since the Recording Secretary would be highest ranking, it would be that person's job preside over the meeting, and the duty of taking minutes would be pawned off to another officer, most likely the Corresponding Secretary. To avoid this convoluted scenario, in 2003 a vote to amend the Bylaws was passed and the two secretaries swapped in rank.