Often you'll hear people talking about lamps wired in series or parallel. The following is a simple explanation of what that means.
The diagram below shows 10 lamps wired in series. In this example, each lamp is rated @ 2.4 Volts. Assuming the lamps are all identical (same voltage and current ratings), the incoming 24V will be shared evenly among them. In that case, the required supply voltage is simply 10 (# of lamps) x 2.4V (lamp voltage). If any lamp is removed (or burns open circuit), the circuit is interrupted and the whole series string will go out.
Some Christmas light strings use lamps that go short circuit when the filament fails. In this case, the circuit continuity is maintained and the remaining lamps stay lit. The down side however is that the supply voltage is now shared by the remaining lamps. In the 10 lamp example, this means each of the 9 remaining good lamps now has 2.66V applied. The higher voltage makes them glow brighter, but will reduce the life of those lamps, possibly causing more to fail. If this happens, the remaining lamps will each see a greater increase in voltage, causing rapid failure. This is why you should replace a defective lamp before the rest of the series string is stressed too much.
For a series lamp circuit, the current drawn from the power supply is the same as the lamp's current rating. If the lamps in our example were rated @ 2.4V and 0.3W each, they would each have a current rating of 0.125A (125mA). Because they are in series, the current drawn from the power supply is still 125mA. The total power draw for the string is 3W. You'll notice that this total can be calculated in two ways. The first is simply the lamp wattage multiplied by the number of lamps (0.3W x 10). The second is the supply voltage multiplied by the lamp current (24 x 0.125).
The diagram below shows 10 lamps wired in parallel. In this example, each lamp is rated @ 12 Volts. That's the same as the incoming supply voltage. If any lamp is removed or burns out (goes open circuit), the remaining lamps will stay lit.
For a parallel lamp circuit, the current drawn from the power supply is the sum of all of the lamp current ratings. If the lamps in our example were rated @ 12V and 0.3W each, they would each have a current rating of 0.025A (25mA). Because they are wired in parallel, the current drawn from the power supply is 0.25A (250mA), while the total power draw for the string is 3W. This wattage total can be calculated in two ways. First, simply multiply the lamp wattage by the number of lamps (0.3W x 10). Second, multiply the supply voltage by the total lamp current (12 x 0.25).
How About Both!
Christmas light strings often use a mixture of series and parallel wiring. Groups of lamps are wired in series, and several of these series strings are connected in parallel. A lamp failure in one series string will not affect the operation of the other series strings.
In the example below, there are 3 series strings of lamps. Each series string requires 24V (2.4V x 10 lamps). As you can see, the ends of each of the 3 series strings are wired in parallel. The series strings are not dependent on each other. Assuming each lamp is rated @ 2.4V and 0.3W, the total wattage of this series/parallel string is 9W (0.3W x 30 lamps). The current drawn from the 24V power would be 0.375W (375mA).
Series lamp wiring can reduce the wire size required due to the lower current. Its main downfall is the inter-dependence of all lamps. One other issue is that all lamps in a series string must be exactly the same current rating, otherwise they will not share the voltage well with blown lamps the result.
Parallel strings do not require that all lamps be the same current rating. They also don't suffer from the "one lamp blows and a whole section goes out" problem. The main drawback is the increased current draw which necessitates thicker wiring. Voltage drop in the parallel cabling can also result in the lamps at the far end of the cable being not as bright as the ones nearest the power feed end.