It's extremely confusing but there are basically 2 measurements systems for food energy:
There's kilocalories (abbreviated as cal) and there's kilojoules (abbreviated as kJ). It can get very confusing because some places will label them calories (cal) and Calories (kJ), lower and upper case respectively which is extremely confusing because 1 kJ is equivalent to 4.81 cal.
According to Wikipedia the US and Canada use kilocalories (cal or calories) and pretty much the rest of the world uses kilojoules (kJ or Calories).
The main difference between the two is that kilocalories are a measure of heat energy, where 1 kilocalorie is the heat needed to warm 1 liter of water by 1 degree celcius. Whereas a kilojoule is a measure of energy usually described by force in newtons.
They're both actually from the metric system, but kilocalorie is the old and obsolete form while kilojoules is the currently accepted metric measurement.