You would have come across word like dual core, hyper threading or may be single core in these time where mobile are quad core at least. Let me explain it the way I understand it my-self. Let consider there is chef in kitchen who is instructed to bake pizza, fry egg and cook tea. Now for time, let name this kitchen Pentium 4. The way he will follow instruction is that he will first bake pizza then fry egg and at last cook tea. The total time he would consume will be a hour for baking pizza, 10 minutes in egg and 5 minutes for tea. In this kitchen 1 hour and 15 minutes will be required for these task.
Now come to another kitchen named Atom (haves Hyper-threading) This kitchen chef is smart because after putting pizza for baking, he start frying egg and soon after cook tea. In this way the tea and egg will be cooked very soon as compare to previous approach or kitchen. In this case total time spent on cooking will be less because while pizza was baking, tea and egg were cooked.
Now let add another chef in this kitchen in this way one would cook pizza and on same time other will fry egg and as soon any of them is free he will make tea. This way kitchen will get free very soon as compared with previous approaches.
Adding of multiple chef can be said as multi core or dual core approach. So let me tell you dual core have two chef and i3 have 2 with hyper threading ability and in i5 there are four chef and in super computer there are millions of kitchen with 8+ or more chef in it.