Every action starting from the database startup and also each and every transaction is recorded in a log file step by step...in case, if within an ongoing transaction, the system crashes or may be for some reason you cannot complete the transaction, then the database will remain in an inconsistent state...so using the log files we can return back to our previous state as if nothing has happened to the database...and also to recover some data log files are used...there are various types of log files like redo log files, event viewer log files etc...all contain the detailed step-by step of the respective process/transaction....
