In essence, you will need one CAL license per user that will access your data and have a "personal" login. This is true for all versions of SQL Server, except Express Edition. With Enterprise and Standard Edition, you can go for CPU license instead.
You can use Express Edition (with the few limitations) without the CALs.
In My WEB application all users are using same user id and password to log int to the DB data. So it's also need to BUY a cal's ? Because my WEB application is host in IIS server.