Plz tell me What a build number represent ? what its purpose ?
like bulid number of XP SP2 2600.2180, W2k3 3790 , LH 5048 etc

Build number is increased with every change - it is common practise for administrators. If you added few lines of code, they wont increase product number, they only increate build number, so they can easily decide which version is newer. Using this it is really simple to know what have been changed since version X...