sp_helpdbfixedrole lists the fixed database roles.
exec sp_helpdbfixedrole
DbFixedRole Description
------------------------ -----------------------------------
db_owner DB Owners
db_accessadmin DB Access Administrators
db_securityadmin DB Security Administrators
db_ddladmin DB DDL Administrators
db_backupoperator DB Backup Operator
db_datareader DB Data Reader
db_datawriter DB Data Writer
db_denydatareader DB Deny Data Reader
db_denydatawriter DB Deny Data Writer