If a capacitor is connected to a battery and is charged, are there charges inside the wires or do they just accumulate on the surfaces of capacitor?
Yes, the surfaces of the capacitor have a net negative or positive charge. The wires are also slightly charged because they also have a very small capacitance, but this is usually neglected.
The ideal capacitor contains charge only in the plates and the charge distribution is uniform. In reality though :
1.The charge distribution is not uniform, the edges having more charge density.
2.The induced field is not perfectly uniform
3.There is always charges on the wires (they carry current) but that does not affect the net capacitence