What are different roles available in SSRS 2008, and what are their purposes?

Item-level roles and system-level roles are the two types of roles  available in SSRS 2008. An item-level role is a collection of tasks related to operations on  an object of the report object hierarchy of SSRS 2008. A system-level role is a collection of tasks related to operations  on server objects outside the report object hierarchy of SSRS 2008.