They are used for executing the reports at a predefined schedule. Although they are two types of schedules, the way their settings are configuring are the same. The difference is in the usage. One can compare their usage to the way the data source are used in the reports.
They are shared by many reports and can be pre configured for usage. Once created they can be used through the subscription page of a report. They can be used to schedule group of related reports at a time. They are typically created by an administrator or content manager as they are shared amongst the reports and require system level authentication.
There is a separate node allocated to handle the shared schedules in the management studio. One can also view reports on the usage of a particular shared schedule through that node.
Like the report-specific data sources they are stored in the report definition itself. They can be used to configure the cache expiration or for timing the generation of report snapshot. They can be created by individual users for their respective reports.