A traditional "bote tamalero" which is a galvanized tamale steamer. The inside bottom is raised with steam slats. Water is placed in the bottom of the steamer, tamales are stacked inside, covered with a lid and steamed for about one to two hours.
While this is the traditional steamers, more people are migrating the the more versatile stacked steamers made with either aluminum or stainless steel.
The traditional versions are getting hard to find. You may find them in a Mexican market. Stacked Steamers can be found in Asian stores and online at GourmetSleuth.com.