The difference between WFE, Application Server and Database Server in a SharePoint Farm

The Web Front end Server is the server that is hosting IIS and serves the websites.
An Application Server  has services on it, for example, like Excel services, Project server, etc..
A database server simply hosts the SharePoint core db's and the other Content databases.. It just has SQL Server installation on it.

So you have to install Sharepoint to WFE's and App. Servers.. You dont need to install Sharepoint to the database servers on your farms.

Removing an Oracle Database from Windows

If you removed a database  but it's services are still showing up at services.mcsi then;
suppose the database name is demodb

oradim -delete -sid demodb

deletes the services, etc. related to this database.

oradim is an oracle utility for windows only!