The Exchange database consists of the following files:
.edb file (rich text database file) contains data placed in the store through the Messaging Application Programming Interface (MAPI), as well as all the database tables that define mailboxes, messages, folders, and attachments.
.stm file (streaming database file) contains common Internet formatted content, such as Multipurpose Internet Mail Extensions (MIME) content, that protocols other than the MAPI protocol place in the store.
.log files (transaction logs) are history files recording server activity. These files are useful in restoring and backing up Exchange data. Transaction logs for Microsoft Exchange server 2000 and 2003 are 5 MB in size. Transaction logs for Microsoft Exchange server 2007 are 1 MB in size.
Each storage group uses its own set of transaction log files. For example, if a storage group contains five stores, all transactions for all five stores are recorded in a single series of transaction log files. You can determine where to locate the transaction log files for each storage group.
.chk (check) files are checkpoint files used for recovering (playing) data from transaction logs into EDB files. The checkpoint is the place marker in the EDB.CHK file that indicates which transactions have been committed. Whenever data is written to an EDB file from the transaction log, the EDB.CHK file is updated with information specifying that the transaction was successfully committed to the respective EDB file. Separate Exx.chk files are maintained for each storage group using ESE (Extensible Storage Engine).