What is Works Contract Service Tax

Hi,

I have purchased a flat last year and it is under construction. My builder has asked me to pay works contract service tax ( @ 4.12% of the total payment made to the builder so far).  I dont know if I have to actually pay this tax.. I have no idea what it means?

Can you explain? Is it legal to levy this tax for all the payments made so far starting from August 2009? Thanks a lot for the information in advance

Hema