Well, this is probably more of a lab question than a starter question, but I'll have a go at giving some answers. At some point, things get more complicated then I understand and a little further on the arguments get purely subjective, so let's see how we go
A capacitor is constructed by putting two conductors in close proximity. Imagine two metal sheets a metre square, and put them a millimeter apart. Connect a wire to each, and you have a capacitor (don't ask me what value, I forgot the math for that a looong time ago
). What happens is that if you apply a voltage to the two sheets, an electric field is set up between them, and you get "positive" charge stored on one sheet and "negative" charge on the other. The bigger the sheets, the more capacitance. The closer the sheets are together, the more capacitance.
If you ever pulled apart an old radio you might have seen a little gadget consisting of two sets of metal fans. The tuning dial caused the fans to move in and out, thus varying the amount of overlap between the two sets. That is a variable capacitor (variable because the effective size of the two sets of sheets varies), and is how the tuning frequency of the radio is varied.
Now, to get two sheets of metal *really* close together, you can't rely on just having an air gap; instead, you put a thin sheet of insulator between them. This is what is referred to as the "dielectric" (more later). Imagine if you put a sheet of ClingWrap between the two sheets and then put the sheets together. They would be insulated from each other, but now they are very close together so the capacitance is very high.
In order to manufacturer these things, you can't be using one-meter square sheets of metal. So you make very thin foil, an inch (say) wide, and you put two strips of it together with some ClingWrap between them. Then you roll them up together (with more ClingWrap), and attached two leads and a label, and sell it for a dollar.
