A Type Ia supernova is different from many others because of the extremely close similarity of the circumstances leading up to it. First you begin with a white-dwarf star which is composed of the Carbon and Oxygen remnant core of a former main sequence star under extreme pressures and held up by electron degeneracy pressure. A white-dwarf which happens to be near enough to a neighboring star may pull matter from that star. This matter will accumulate on the surface of the white-dwarf and increase its mass over time.
Eventually, if the mass of the white-dwarf reaches very near to the Chandrasekhar limit of about 1.4 solar masses the pressure and temperature of the star will start to rise to the point where Carbon and Oxygen fusion begins. This fusion process proceeds extremely rapidly. In a normal star the increase in temperature due to fusion reactions would result in expansion due to increased pressure, but a white-dwarf star is composed of electron degenerate matter which won't expand due to temperature in the same way. The fusion reaction has no means of regulating itself and thus rapidly progresses to the point of releasing enough energy to tear apart the star in only a few seconds. After the brightness from the initial explosion has diminished the supernova remnants will continue to shine for many days due to the radioactive decay of the Nickel-56 produced via fusion.
Most likely, due to the similarity in conditions leading up to the supernova (a Carbon/Oxygen electron degenerate core with almost exactly 1.4 solar masses of material around it) Type Ia's all have a very similar peak luminosity and relationship of luminosity to time.
However, they are not 100% identical. Variations in the composition of the white-dwarf (due to variations in the composition of the main sequence star that preceded it) will affect the supernova luminosity, but this variation can be accounted for by carefully observing the light-curve.
Because Type Ia supernova have very characteristic light-curves, because they are so bright and remain substantially bright over a period of days they can be used as "standard candles" across the entire Universe. SN 2011fe in M101 is the closest Type Ia supernova to have ever been observed with modern instruments. Being able to study the light-curve and spectra in enormous detail will help us improve our models of Type Ia supernova and perhaps lead to new insights or at least enhanced precision related to using them as standard candles.