The script give you the list of all users and their respective Server role.
select su.name,p.name from sys.server_principals P
INNER JOIN sys.server_role_members sr
On P.principal_id = sr.role_principal_id
INNER JOIN (Select sr.Name,p.principal_id from sys.syslogins sr INNER JOIN sys.server_principals p
On P.sid = sr.sid)su
On sr.member_principal_id = su.principal_id
This article covers four of the fixed database roles (db_datareader, db_datawriter, db_denydatareader, and db_denydatawriter). If you're new to SQL security (and maybe even if you're not) this article is worth reading.