A read-only file or storage device contains data that cannot be modified or deleted. While data can be accessed or "read" from a read-only file or device, new data cannot be added or "written" to the device.
Most operating systems, such as Windows and OS X allow you to mark individual files as read-only. In Windows, you can right-click a file and select to view more information about the file. You can click the Read-only checkbox (located in the General tab) to make a file read-only. In OS X, you can right-click a file and select to view more information about file. You can then select the Locked or Stationery options in the file's Get Info window to disable write access to the file. A "locked" file on a Mac is the same thing as a read-only file in Windows.
Windows and OS X also allow you to set folders or entire storage devices to read-only. In some cases, the storage media itself may be read-only. For example, DVDs and CD-Rs are permanently read-only once the initial data has been written to the disc. Some flash drives and SD cards have a physical switch that can be used to make the media read-only. By setting a storage device to read-only, you can ensure that important data isn't accidentally modified or erased.