Priyanka,
I think you would benefit by reading some of the older DFD text books about. Now Ed Yourdon, being the gentleman that he is, has made one of his older and still relevant books available on the internet in a Wiki. see http://www.yourdon.com/strucanalysis/index.html
When you read this text, you'll understand what was meant by things like "the essential data flows - that are technology independent"
There was a UK training company that had a tutorial on their website about using DFDs and Use-cases, if and when I remeber, I'll post it.
warm regards,
K