Difference between read and write permission in windows

Whenever you are currently using a file(e.g. playing an audio or video file) check the properties before and after the usage by right click or pressing Alt+Enter) when the file is highlighted.You can notice that the read only property of that file is automatically marked and you are not able to delete that file at that time but still you can copy it to USB etc. and also you can open it with other application this is because copy or play option requires only read permission but delete operation requires write permission which is not available when read only is marked.Therefore you are not able to delete a file when it is currently in use and most of the viruses uses this concept to make the virus file undeletable.At that time you can change the properties of a specific file from the command prompt using attrib command to change the permission and delete that file.