Energy production and use is one of the great dilemmas of our time.
There are two types of energy - renewable and non-renewable. Renewable energy is taken from sources which will always be around - solar energy from the sun, wave and tide energy from the oceans, wind energy, geothermal and even things like biomass (plants and trees) that either regrow by themselves or with a little help from humans. Non-renewable energy is taken from sources that don't have the ability to regenerate anywhere near as quickly as we are using them. Examples of non-renewable energy are coal, petrol, diesel and oil, natural gas and uranium (which have all been produced over many millions of years).
Light, heat, fuel oil (eg petrol / diesel), and electricity are types of energy that most of us use every day in our modern lives.
Light energy from the sun makes it light during the day, and electric light energy helps us to 'extend the day' by turning on streetlights and household lights.
Heat energy also comes from the sun in the form of radiation. This is why we feel warm when we stand in the sun, and why summer is warmer than winter (the sun is closer then so we get more heat energy). We use electric heating and other forms of stored energy (like wood and coal) to heat our homes and workplaces.
Fuel oil (petrol, diesel and lubricating oils) are made from petrochemicals, which are the remains of massive amounts of dead algae that died many millions of years ago, sank to the bottom of the oceans, got covered in rock and sand and squeezed and compressed - eventually turning into oil. We use fuel oil energy for controlled combustion in engines to power our cars, trucks, tractors and machinery. Fuel oil is one of the most expensive forms of energy, because it requires a lot of work to find it and access it - often, it's still deep under layers of rock beneath the oceans - not very easy to get to! As a society, we're heavily dependent on fuel oil energy for our transportation in particular. There are fledgling efforts to replace fuel oil with other affordable forms of energy that're renewable like bio-diesel made from plants and electric vehicles with electricity being produced from renewable resources. These new technologies aren't able to provide all the energy needed to replace fuel oil yet, but they will increase in availability over the years as fuel oil becomes even more difficult to safely extract from deep within the earth.
In New Zealand, we're pretty lucky - our electricity production comes predominantly from renewable resources - around 73% of total electricity production in fact. But still, we rely on coal, diesel and natural gas for the remaining 27% of our energy use. In order to have sustainable electrical energy production nationwide, we need to achieve 100% of our electricity generation from renewable resources only.