What key points are to be noted down while studying functional flow of the application...

For understanding one application and making another application based on that you need to tack care these many things.

First of all Sandeep is right, Oracle is database and .NET if framework or technology which will allow you to create front-end side for your application like web form or desktop form, while Oracle provide back-end side for storing data to database. Also Microsoft SQL server is same as Oracle to store data in database.

you say you have been provided desktop application and you are going to make web application.

1) Desktop application faster than your web application because it will directly run from your machine while web application is run over internet or intranet so it will somehow slow based on network connectivity.

2) Some functionality works on desktop while some are not possible on web so you need to tack care for that, and also need to find alternate for that functionality to make work on web.
3) You need to check each and every form of desktop application and need detail study for functional area behind that, so you can easily understand what you need to do while developing your web application.

4) For this you can make one document for functional flow and write down each and every point there so it will very help full to you as well as others to understand the functional area of application.

These are the major thing you need to tack care while understanding an application functional flow.