Details aren't clear, but this appears to be an agreement between Texas A&M and the NCAA. One that, as Brett McMurphy puts it, "closes the book" on the NCAA's investigation into whether Manziel accepted money to sign autographs—with the NCAA clearing him of that charge.
Liucci speculates that Manziel's violation could be a minor one, suggesting bylaw 220.127.116.11, which requires players to "take steps to stop" others making money off them "without the student-athlete’s knowledge or permission." So: a plea deal in everyone's best interests. Just don't tell Dez Bryant.
Update: Here's the official statement from the NCAA. It's a school-imposed suspension, and the NCAA has accepted it.
Texas A&M University and the NCAA confirmed today that there is no evidence that quarterback Johnny Manziel received money in exchange for autographs, based on currently available information and statements by Manziel.
The violation was declared "inadvertent," and an an A&M administrator told USA Today the same thing. The story seems to be that Manziel signed stacks of autographs but didn't realize they would be sold, and no proof was uncovered that he received money for signing.
More on the NCAA's reasoning:
18.104.22.168 Advertisements and Promotions After Becoming a Student-Athlete. After becoming a student-athlete, an individual shall not be eligible for participation in intercollegiate athletics if the individual:
(a) Accepts any remuneration for or permits the use of his or her name or picture to advertise, recommend or promote directly the sale or use of a commercial product or service of any kind;
or (b) Receives remuneration for endorsing a commercial product or service through the individual’s use of such product or service.
22.214.171.124 Use of a Student-Athlete’s Name or Picture without Knowledge or Permission. If a student-athlete’s name or picture appears on commercial items (e.g., T-shirts, sweatshirts, serving trays, playing cards, posters) or is used to promote a commercial product sold by an individual or agency without the studentathlete’s knowledge or permission, the student-athlete (or the institution acting on behalf of the student-athlete) is required to take steps to stop such an activity in order to retain his or her eligibility for intercollegiate athletics. Such steps are not required in cases in which a student-athlete’s photograph is sold by an individual or agency (e.g., private photographer, news agency) for private use.
The first sort of contradicts the second? 126.96.36.199 would apply for someone who knew about and permitted use of his name for commercial gain. 188.8.131.52 applies if the athlete has no idea someone is making money off of his image. Also, the fact that bylaws can now be violated in "spirit" is an odd precedent.