SQL Server Level Roles are server-wide, not database wide in scope. They are as follows:
sysadmin – keys to the castle. You can do ANYTHING.
serveradmin – not as powerful as sysadmin, but still pretty potent. You can shut down SQL, and change server-wide configuration settings (sp_configure)
securityadmin – can create and manage SQL server logins and their properties. Also, if a member has access to a specific database, they can grant\deny\revoke permissions within that database. MS recommends that you treat securityadmin as equivalent to sysadmin.
Processadmin – can kill processes within a SQL server instance, (KILL spid)
Setupadmin – Should be called linkedserveradmin. Can set up and removed linked servers
Bulkadmin – Can run BULK INSERT
Diskadmin – can manage disk files, like sp_adddumpdevice
Dbcreator can create ALTER AND DROP databases.
Public – EVERYONE – Be very careful about what rights you grant to public.
References:
BOL: http://msdn.microsoft.com/en-us/library/ms188659.aspx