Permissions of SQL Server Fixed Database Roles

Fixed database roles can be mapped to the more detailed permissions that are included in SQL Server. 

Fixed database roles are provided for convenience and backward compatibility. 

All fixed database roles are granted with Server-level permission VIEW ANY DATABASE.

Fixed database role	Database-level permission
db_accessadmin	Granted: ALTER ANY USER, CREATE SCHEMA Granted with GRANT option: CONNECT
db_backupoperator	Granted: BACKUP DATABASE, BACKUP LOG, CHECKPOINT
db_datareader	Granted: SELECT
db_datawriter	Granted: DELETE, INSERT, UPDATE
db_ddladmin	Granted: ALTER ANY ASSEMBLY, ALTER ANY ASYMMETRIC KEY, ALTER ANY CERTIFICATE, ALTER ANY CONTRACT, ALTER ANY DATABASE DDL TRIGGER, ALTER ANY DATABASE EVENT, NOTIFICATION, ALTER ANY DATASPACE, ALTER ANY FULLTEXT CATALOG, ALTER ANY MESSAGE TYPE, ALTER ANY REMOTE SERVICE BINDING, ALTER ANY ROUTE, ALTER ANY SCHEMA, ALTER ANY SERVICE, ALTER ANY SYMMETRIC KEY, CHECKPOINT, CREATE AGGREGATE, CREATE DEFAULT, CREATE FUNCTION, CREATE PROCEDURE, CREATE QUEUE, CREATE RULE, CREATE SYNONYM, CREATE TABLE, CREATE TYPE, CREATE VIEW, CREATE XML SCHEMA COLLECTION, REFERENCES
db_denydatareader	Denied: SELECT
db_denydatawriter	Denied: DELETE, INSERT, UPDATE
db_owner	Granted with GRANT option: CONTROL
db_securityadmin	Granted: ALTER ANY APPLICATION ROLE, ALTER ANY ROLE, CREATE SCHEMA, VIEW DEFINITION
dbm_monitor	Granted: VIEW most recent status in Database Mirroring Monitor Important The dbm_monitor fixed database role is created in the msdb database when the first database is registered in Database Mirroring Monitor. The new dbm_monitor role has no members until a system administrator assigns users to the role.

For more info: http://msdn.microsoft.com/en-us/library/ms189612(v=sql.105).aspx