There are four forces of nature known. These forces are responsible for mediating the interaction between the elementary particles and everything we know of.  Much like Maxwell's unification of the electric and magnetic forces to electromagnetism the four forces are thought to be the broken symmetry of one force.

    The standard model is based on a Lie group called SU(3) X SU(2) X U(1). The matter particles consist of three families of quarks and leptons. ( There are also          addition matter particles called ``Higgs particles'', which are required to account for the fact that part of the symmetry is spontaneously broken.

The standard model contains some 20 adjustable parameters, whose values are determined experimentally.
     
Gravity is not  produced from the Standard model.

Force	Range	Mediating Particle	Spin
Gravitation	Long	Graviton	2
Electromagnetism	Long	Photon	1
Weak Nuclear Force	Short	W+, W-, Z bosons	1
Strong Nuclear Force	Short	Gluons	1

So far only the  Weak Nuclear force and the Electromagnetic force  have been unified. 

This was accomplished by  Glashow, Salam, and Wienberg.

They did this by showing that in the gauge theory symmetry can be broken at low energies to show the differences between the weak and electromagnetic forces .

Using this idea it is thought that the forces of nature are indistinguishable at very high energies. 

Gauge theories that atempt to show this unification are called Grand Unified Theories or GUT

This graph shows the unification of the forces at about 10^1 9 GeV

[9]                                                   
                                                                                  SU(3) = strong force

                                                                            SU(2) = weak force
                                                                             U(1) = electromagnetic
                                                                           SU(2) x U(1) = elctroweak

Grand unification ran into difficulties too. Extrapolating a theory to very high energy scales poses a problem sometimes called the "hierarchy problem". This can be loosely stated as follows: if the various forces in a theory become unified at a very high energy scale, then this "natural" energy scale is much higher than the masses of familiar elementary particles such as electrons and quarks. In such a situation, one has to explain why these particles are so much lighter than the natural energy scale of the theory .(about ten orders of discrepancy )