You're right, C and D batteries both are 1.5 volts. Actually, AA and AAA are also 1.5 volts. That makes you wonder what the difference is. If they're the same voltage, why not just use all AAA batteries and make things lighter and smaller. But there is a big difference between them all.
In an electrical circuit or device, there are 2 really important things. One of them is voltage. The other is current. The classical way you hear them talked about is by comparing them to a pipe with water in it. The voltage is like the water pressure which pushes things down the pipe. And current is like the amount of water flowing down the pipe. Some electronics need lots more current but don't need more voltage.
That's where the batteries come in. The D cell batteries can give more current than the C batteries. If you needs lots of current and try to get it from a tiny battery, it won't work as well.
As for adding them together, you'll notice that in most electronics, you put them in so that the positive end of one touched the negative of the other. That adds the voltages of the batteries. This is called adding them "in series". So if you have 2 D batteries in series, you have 3 volts.
(republished on 07/13/06)