The Bradford assay is a colorimetric assay based on the interaction between Coomassie brilliant blue (you know, the stuff you stain your gels with) and the arginine and aromatic residues in your protein. When the dye binds to these residues, its maximum absorption shifts from 470 nm to 595 nm. In general, you measure the absorbance of a series of known concentrations of a standard protein, generally BSA, and create a standard curve. You then use that standard curve to calculate the concentration of your protein sample based on its absorbance.