I know how to develop and deploy a Salesforce app but I am still a bit sketchy about some aspects.

I am developing an application which has certain user previlages for certain users. (Can this be achieved?)
Can different users have different user screens according to the previlages?

1. I have observed there are certain tabs like Dashboard under the Apps. Can these be kept hidden from the users for lesser confusion?
2. Can these applications be via an extension to the organisation's url itself instead of salesforce?                                                                                                  If "NO", Why? Can someone please explain?
3. Does anyone who would like to use the application from my organisation have a Salesforce account?                                                                                        If "YES" what type of Salesforce account would that be?
4. To use the app,if someone accesses the link where the app is hosted, Should the user be complusarily logged into their Salesforce Account?                                                                                                                                                                      

Please can someone address my issues?

I know how to develop and deploy a Salesforce app but I am still a bit sketchy about some aspects.

I am developing an application which has certain user previlages for certain users. (Can this be achieved?)
Can different users have different user screens according to the previlages?

1. I have observed there are certain tabs like Dashboard under the Apps. Can these be kept hidden from the users for lesser confusion?
2. Can these applications be via an extension to the organisation's url itself instead of salesforce?                                                                                                  If "NO", Why? Can someone please explain?
3. Does anyone who would like to use the application from my organisation have a Salesforce account?                                                                                        If "YES" what type of Salesforce account would that be?
4. To use the app,if someone accesses the link where the app is hosted, Should the user be complusarily logged into their Salesforce Account?                                                                                                                                                                      

Please can someone address my issues?