NZB Download Checker is a small freeware tool that checks an NZB for you, in order find out if the files you want to download from Usenet exist on the newsserver(s). This way you don't have to download large files, only to find out half way through, you won't be able to get the download complete and are wasting your time and bandwidth.
Every file on Usenet is split up into many separate parts and every part has its own unique number (Message ID). NZB Download Checker sends all the Message ID's listed in the NZB file to your newsserver(s) and checks which reply it gets back from the newsserver. This way it can determine how much of the data is available without actually downloading the files.