What are roles and how are they used in database security and permissions?  Provide an example of a predefine server role.

What will be an ideal response?

A role groups together permissions on different database objects. A role can be granted to an individual user, known as a member in the context of the role, and that user will inherit the permissions granted to the role. Roles should be used whenever possible to manage permissions for groups of database users with similar access needs. SQL Server 2012 has several predefined roles at both the server and database level. An example of a server role is the sysadmin role, which can perform any function on the database server. An example of a database role is the db_owner role, which can perform any function for an individual database. Roles can also be used to model a business function to differentiate different access levels that groups of users need at a granular level.