Exchange is a transaction based messaging system. It writes new transactions in 4kb pages into memory. Thus the database is never truly in sync. while exchange is running. These are called "dirty" pages, and are only written to disk (information store) when exchange is stopped and the pages are flushed.

You can imagine what would happen if a sudden power loss was to occur. All the inconsistent "dirty" pages would be lost. Thus, data would be lost. In order to maintain integrity, all transactions are written to a transaction log. i.e. edbxxx1.log. Once a log reaches 5 MB it rolls over to the next edbxxx2.log, and continues. This is how exchange mitigates failures. Many people treat the transactions logs, as more important than the information store itself.

-Quad