Let's say you start with 1 liter (roughly 1 kilogram, 2.2 lbs) of cold water at about 10°C (50°F) and you want to raise it 90°C to its boiling point (100°C or 212°F). The amount of energy you need is 4.2 × 1000 grams × 90 degrees = 378,000 joules or 378 kJ.
The mysterious "4.2" is a constant value called the specific heat capacity of water. Every material has a different specific heat capacity, which is simply the amount of energy you have to put in to raise the temperature of one gram of the material by one degree centigrade. You need to add 4.2 joules of energy to raise the temperature of 1 gram of water by 1°C, so water's specific heat capacity is 4.2 J/g/°C.
378kJ to boil a liter of water is rather more energy than you might think. An energy-efficient lamp rated at 10 watts uses 10 joules of energy every second (because 1 watt means using one joule per second), so it would take it 37,800 seconds—about 10.5 hours—to use as much energy as our kettle uses in a single boil!
If you're using an electric kettle rated at 2400 watts, that means it's consuming 2400 joules of electrical energy per second and putting (roughly) the same amount of energy into the water as heat each second as well. Divide 378,000 by 2400 and you find it takes the kettle about 160 seconds to do the job, which sounds about right—an electric kettle generally does take about 2-3 minutes to boil. An old proverb says a watched pot (kettle) never boils, but that dates from the time when most people used to boil water on hideously inefficient open coal fires. An electric kettle can boil water in just a couple of minutes because it can add heat energy to the water much more quickly and efficiently than an open fire (which allows heat to escape in all directions).