Here http://www.ams.org/publications/notices/whatis/noticesarchive you can find a list of short "What is" type AMS notices articles. At the same page you can select an year and month and look for the corresponding issue's table of contents. For a list of other journals which publish expository papers, see the answers to this MO question http://mathoverflow.net/questions/15366/which-journals-publish-expository-work