MAA is an organization devoted to the improvement of the teaching of college mathematics. Carleton College is an institutional member of the MAA. The North Central Section of the MAA holds a spring and a fall meeting each year and many of the papers presented at these meetings are of interest to undergraduates.
Students who are planning a professional career in mathematics may well want to become members of the MAA while they are still in school. The MAA sponsors the famous Putnam Exam that is given each December. The department honors a small number of students each year by nominating them as members of the Association (and paying their dues).
Finally, the MMA publishes the American Mathematical Monthly and Mathematics Magazine, two excellent journals.
The principal goal of the American Mathematical Society is to support research in mathematics. The AMS publishes Mathematical Reviews, the Bulletin, the Proceedings, and other research journals and monographs.