OST or Offline Storage Files represent the user's mailbox data in offline mode on the local machine upon registration with Exchange Server using Microsoft Outlook. It is automatically created on the first use of Microsoft Outlook upon connectivity with the server. Once the file is created, the data is synchronized with the email server so that it is available offline as well in case of disconnectivity from the email server.
A JPEG is a type of image format that is saved using the method of lossy compression. The output image, as result of compression, is a trade-off between storage size and image quality. Users can adjust the compression level to achieve the desired quality level while at the same time reduce the storage size. Image quality is negligibly affected if 10:1 compression is applied to the image. The higher the compression value, the higher the degradation in image quality. JPEG image file format was standardized by the Joint Photographic Experts Group and, hence, the name JPEG. The format has been the choice of storing and transmitting photographic images on the web. Almost all Operating systems now have viewers that support visualization of JPEG images, which are often stored with JPG extension as well. Even the web browsers support visualization of JPEG images.
You can also convert OST into many other file formats. Please see the complete list below.