The difference between Calories and kilojoules is very much the same difference between metres and feet.  They are just two different measures of the same thing, energy.  It can be quite confusing though as often food is measured in kilojoules and exercise in Calories.  This makes it tough to compare the two, but let's have a look at how it all works:

1 Calorie = 4.184 kilojoules

(note that Calorie is with a capital 'C' as this actually means 1000 calories, which is the way we are used to seeing it) 

Often you will want to convert kilojoules into Calories (or vise versa) and so the easiest way is to just divide the kilojoules figure by 4 to get a rough figure.  If you want to be exact you need to divide by 4.184.  

So in an example 2343 kilojoules = 2343/4.184

                                                             = 600 Calories

Remember that both Calories and kilojoules are both just measurements of energy.  We bring energy into our body through food and burn it through exercise and our basal metabolic rate (breathing, blood circulation & basic bodily functions).  When you consume more energy than your body burns, the excess energy gets converted into fat and stored in your body.  On the other side when you expend more energy than you eat your body burns excess stores of fat and this is how you lose weight.  This should help you understand why it is so important to monitor both your calorie intake and expenditure.  All you need to do is make sure you burn more energy than you eat, and you WILL lose weight.   

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