from Books Online roles-SQL Server, overview
Fixed server role Description :
sysadmin Can perform any activity in SQL Server.
serveradmin Can set serverwide configuration options, shut down the server.
setupadmin Can manage linked servers and startup procedures.
securityadmin Can manage logins and CREATE DATABASE permissions, also read error logs and change passwords.
processadmin Can manage processes running in SQL Server.
dbcreator Can create, alter, and drop databases.
diskadmin Can manage disk files.
bulkadmin Can execute BULK INSERT statements.
You can get a list of the fixed server roles from sp_helpsrvrole, and get the specific permissions for each role from sp_srvrolepermission.
Each database has a set of fixed database roles. While roles with the same names exist in each database, the scope of an individual role is only within a specific database. For example, if Database1 and Database2 both have user IDs named UserX, adding UserX in Database1 to the db_owner fixed database role for Database1 has no effect on whether UserX in Database2 is a member of the db_owner role for Database2.
Fixed database role Description:
db_owner Has all permissions in the database.
db_accessadmin Can add or remove user IDs.
db_securityadmin Can manage all permissions, object ownerships, roles and role memberships.
db_ddladmin Can issue ALL DDL, but cannot issue GRANT, REVOKE, or DENY statements.
db_backupoperator Can issue DBCC, CHECKPOINT, and BACKUP statements.
db_datareader Can select all data from any user table in the database.
db_datawriter Can modify any data in any user table in the database.
db_denydatareader Cannot select any data from any user table in the database.
db_denydatawriter Cannot modify any data in any user table in the database.
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