All of the religious objects and decorations in a Zen temple are designed to remind us of the deeper purpose for which we train: realizing—and living in harmony with—our True Nature, our Buddha Nature. Religious objects are spiritual "tools" that help us shift our focus from ourselves to That which is greater than ourselves.
The main altar in a temple contains objective symbols that represent aspects of the Eternal that we aspire to emulate, and also offerings that express our gratitude and joy in training. A temple will also have side altars dedicated to the Bodhisattvas Avalokitesvara (Great Compassion) and Acalanatha (our will to train ourselves).
The thumbnail photos above link to pages containing descriptions of the various altars and their contents.