They have certainly found something that is very close to the Higgs Boson predicted by the standard model of particle physics. How do they know its the Higgs?
> The standard model gives very specific predictions for the behavior of the Higgs, which are fixed by the distinctive properties of the Higgs field (spontaneous breaking of the SU(2) gauge symmetry fixes how it interacts with gauge fields, giving mass to fermions fixes its interaction with them). These predictions have for a long long time shown that if the Higgs mass is 125 GeV, it could not be observed until the energies and luminosities of the LHC were available, and then it should be seen with certain specific signal sizes in certain specific decay channels.
> What the LHC experiments are seeing is, after decades of no such signal in these and similar channels, signals appearing in just the right channels, with roughly the right signal size. If this isn’t a Higgs, it’s something very like it.
We don't yet know if whatever we see is or is not the SM Higgs. That's why the LHC is scheduled to continue running through the year, so we can get more details. All we can really say at this point is we see something. Heck, we've been able to say that from December!
http://blog.vixra.org/2012/06/24/higgs-discovery-on-the-brin...
There are some intriguing unexpected signals in the data that is public so far:
http://blog.vixra.org/2012/06/29/whats-the-deal-with-h-%E2%8...