'My Folders' is your own folder. It means it is specific to each user. when you will create your report under this folder then no one in the application can see and access these reports because these are saved and created under your My folder. so this is totally yours personal Development.
Shared Folders is your Application Common Folder. It means it is shared across users of the application as per the roles. Shared folders have many Sub-folders related to the Work related and Module Related.
Oracle has already given standard OTBI reports, Data Models and Dashboards under the sub-folders in the Shared Folders. When you will create any report under the sub-folder of Shared Folders then your report will be visible and accessible to other users of the application as per the Permission and role of that user.