what is .ost file?

Offline folders (.ost) files are stored on your computer and are available even when the network is down. The default location for the .ost is your current Windows folder. Like personal folders (.pst) files, .ost files can be compacted to save space on your computer. The difference between an .ost file and a set of .pst files is that the .ost file starts as a mirror image of your folders on the Exchange Server, and works in conjunction with the Exchange Serve during synchronization. A set of .pst files, on the other hand, is simply a storage location on your hard disk or a server other than the Exchange Server.