A DC circuit is a circuit in which the voltage and current throughout the circuit are constant. For a simple DC circuit, as shown in the picture to the left, all of the elements need to be connected. For example, if one of the light bulbs in the circuit was unscrewed, then the other light bulb would go out, and the whole system would stop. Throughout this circuit, the voltage and current is constant. So if the battery was a 5 volt battery, the voltage throughout the entire system would be 5 volts. The same idea applies to the current. The current running through the entire circuit is constant, meaning that if you used an ammeter to measure the current throughout the circuit, then you would find that the current is constant.
Another type of circuit is the parallel circuit as shown to the left. In this circuit, one of the light bulbs could be unscrewed and the other would still be lit. In this circuit, the voltage throughout is constant just like in the circuit above. The current, however, is a little different. In each light bulb the current is half of the total current in the circuit, meaning that when the currents in the parallel are added their sum is equal to the total current in the circuit.
The circuit shown to the left is a more complex circuit which includes two light bulbs in a parallel circuit in a series with another light bulb. So if one of the light bulbs in parallel was unscrewed the others would still be lit, but if the third light bulb was unscrewed, all of the lights would go out. The voltage throughout the entire system is constant as it is in the two situations above, but the current, however, is not. The current running through the single light bulb is equal to the total current and the current through the bulbs in parallel is half that of the total current like in the parallel circuit above.