Physicists study heat to understand how things act at different temperatures. Heat is a form of energy and temperature measures how much energy an object has. The study of heat is really the study of the atoms and molecules that make up an object. The faster the atoms are moving, the hotter the temperature because they have more energy. (The picture shows kids behaving like hot atoms.)
Heat can be made in many ways. One way is burning. Here, the chemicals of the burning object change into other chemicals and release energy in the process. Heat can also be made by friction (rubbing). Try rubbing your hands together and notice how they get hot. In both of these cases, the atoms and molecules are moving around more when they heat up.
In the US, we measure temperature in degrees Fahrenheit (oF). Water freezes at 32o F and boils at 212o F (at sea level). In most other countries they measure temperature in degrees Celsius (oC). In Celsius, water freezes at 0o C and boils at 100o C. Scientists use Kelvin (K) to measure temperatures.
When we take heat energy away from something we make it cold. The coldest anything can get is -460o F (-273o C or 0 Kelvin). This is called absolute zero. At this temperature almost everything is frozen solid, even the air we breathe. On the other hand, things can theoretically get as hot as they like. The hottest things we know of are stars. They have temperatures of many millions of degrees.