When to use SharePoint

Hi,

I want to develop an intranet web-application which must be able to do some
basic things like leave request, HR mngt, time sheet... I know these are posible
in SharePoint but what i want to know is how easy is it to do.. As i am new to
SharePoint i want to know for such or even more complex application what i can
chose (ASP.NET or SharePoint Services). Can i develop same applications on
ASP.NET and SharePoint? What about the database will i be able to use ADO.NET
the same way as i use in ASP.NET? I read that each List in WSS is a table in SQL
but can i use relations how can i define Table Relations?? Please Help Me...