Forces War Records as the name suggests lists millions of records specifically related to the military. As they are a niche site they can cater far better than the general sites to military records and with each name record you generally get a lot of supplementary data, such as information on the unit they individual may have served in, or the ship they served on, information on medals awarded, battles fought in etc.
No site lists every individual ever born and Forces War Records is no different. However, they do have a very good reason for this. Approximately 50 million or so individuals have served since the 1800′s, but most of the time only officers were recorded unless an individual died in battle or was awarded a medal. In addition to this the British and German did a good job at destroying lots of records in the 2nd World War, and what ou are left with is a possible few million records. At last count they had about 5 million actual name records.
If we have one criticism of the site, it is that it can take a while to drill down to the specific person you are looking for before registering if you don’t have their service number. It’s a small complaint I know, but not many sites deal with this very well IMHO.