I have some basic understanding problem about how HDL works.I am too good at digital systems and microprocessors(and its role as replacement of digital logic.)
I MEAN ALU and basic flip-flops.i HAVE ALSO EXTENDED MY KNOWLEDGE UPTO INTEGRATED CIRCUITS AND DIGITAL ELECTRONIC CIRCUITS.(logic families etc)

But I am stuck at very basic understanding of HDL. I have an integrated circuit which has been integrated on large scale basis.so how will I use HDL like Verilog ?? What is exactly that ?

Here I have found some basic information which helps.

Initially VHDL is used for documentation purpose. To realize big circuits say with 500K or more gate count in schematics is difficult. even a small change in the circuit is difficult. HDL is H/W programming language using the synthesizable constructs, circuit can be realized and verified. Since HDLs are technology independent they can be mapped to any technology(90nm, 65nm...) without much effort.

HDL is an Hardware Description language, which comes in two main flavors VHDL and Verilog.

Assume you want to build a 32bit ALU of your own logic.Of course it can be done using your basic gates ICs(7400,7402), But there are few constraints in it like timing issues,power consumption etc..
Now coming to an FPGA and HDL combination you can write a code of ALU and describe how should it work.
regards
Kiran

goto http://esd.cs.ucr.edu/labs/tutorial/

this may be helpfull...