Building Custom / Dynamic / Interactive Forms

Hello I am trying to build  a simple form with following features.
1. If a specific drop down selected, form controls changes accordingly ( some text boxes will hide , some will become visible.
2. Form notification email goes to selected drop down category and Cc to the submitter.
Its pretty straight forward in .net but I am not sure if I have to use Form builder or API in SF. I will appreciate any help.

the form builder is not a requirement, it's just there to make it easier to build simple input forms and log the results.

It might be possible to work with the Form Builder to accomplish this, but if you only require emails to be sent (and not logged in the Form Builder) I personally would find it faster to build a regular ascx user control. That way I can have complete control over the inputs, and use jquery to show/hide fields, and finally send the email to whomever.

This is possible with the form builder, but by default, notifications are subscribed to, so if you want to send a custom email based on the form, you'll probably have to override the control to subscribe to the "Submit" action and send the email manually anyway.

If you do need the Form Builder to log responses, you could use the API in your custom ascx control to populate the results.

I hope this is helpful

Thanks..
I also need to log the entered submissions. I am familiar with developing custom ascx controls, but how can I build for SF, is there any documentation link,  just to get start with .