As DB's said already, elements that exhibit multiple valency form more than one compound with another element (as long as the two will react). The dominant valency leads to the more stable compound. Less favored valencies (or oxidation states) result in less stable, or more reactive compounds.
In tha case of Au, the +3 state is much more stable than the +1 state (the reason for this is a little complicated). But gold itself is quite an unreactive metal, so it will react only with the most electronegative elements like the halogens (F, Cl, Br, I) and the chalcogens (O, S, Se, Te).
Since the +3 state is preferred, compounds like AuF3, AuCl3, AuBr3, Au2O3 are quite stable. However, AuCl, AuBr, Au2O also exist, though they are less stable. But when you get to the bigger atoms (Se, I), it becomes difficult to squeeze 3 of these around a gold atom. The repulsion between these atoms (also known as steric hindrance) becomes very strong. So, with the bigger atoms, the +1 state is preferred, and so, in these cases AU2Se and AuI are the more stable compounds.
This is just a crude explanation, and the details are far more complex. But the general lesson to be learned is that, when an element exhibits multiple valencies (this is particularly pronounced with the d-block, or transition metals), it will form multiple compounds with another element, their stabilities generally depending on the dominance of the valency.
