You can see users idd on each database where they have rights to,
You wil see the SQLServer users and the WindowsUser if any,
Or you can even see Windows User Groups on these tabs, and SQLServer Groups
The great difference:
SQl server uses userlogins on Server Based niveau (witch tells whether you may access the server),
Then once a user has access to a server this does not mean he also have access to different databases / tables /views or any, they have to be declared .
The easiest way for management purposes is to create windows user groups in your Active Directorie and use these in your SQL Server environment,
This way you can declare access to a specific database / table for a certain "usergroup", otherwise you end up setting individual rights for every user.
Ps: you can also put a windows user/usergroup in a SQLServer group (db_datareader,..)
Schema's is not my thing but i try, please correct me when im wrong.
Schema's are something where you can divide ownership in a certain database,
take for example a database called "TEST", when created by the sa this would mean that every table created in in database TEST would be prefixed by dbo.tablename
so you get in full syntax:
but here you can create schema's on serverniveau like ex. schema "finance"
put the user "jan" as schema owner for this schema
now you could create a table like
SQLSrvName.TEST.Finance.janstable in witch the user jan would be the owner
i hope this helps you further.