When I was young light strings came in two varieties, those that went out all at once when just one lightbulb burned out, and those that when a lightbulb burned out only that one bulb went dark. Of course back in those days all lights had filaments, and the current through a thin tungsten wire caused the lightbulb to glow.
The light strings that went out all at once were wired in series, where the current flowed through the lightbulbs one after the other. If a filament burned out the electrical path would be broken, and the whole string would go out. The ones that operated independently were wired in parallel, so the current passed through each light bulb from one side of the circuit to the other independently. If one lightbulb burned out the rest were still passing current, so they would still light up.
All of the lightbulbs had an appearance to mimic that of a candle, a glass shape that was widest near the base and tapered towards the tip. The ones that went out all at once were narrower than the others. And, the color of the ones that went out one at a time wore their colors thin and showed spots after years of use.
The bulbs were not interchangeable, since they had different size sockets that they had to fit.
Introduction Image: Yoda light set on Amazon. This product is shown below.