Things you need to know.
What is temperature?
Temperature is the average amount of energy of the motion of the particle of a substance. You can see this happen in boiling water, the water molecules vibrate at high speeds and the bubbles in the water is escaping heat. We measure temperature by the presence of heat energy or the lack thereof. We measure heat with a thermometer, These are a glass tube with usually mercury or colored alcohol, this works because liquids expand as they are heated and how much they swell allows us to know the temperature.
There are several scales (ways to measure) of temperature. The two most common are the Celsius and Fahrenheit scale. On Celsius the freezing point of water is 0 degrees celsius and the boiling point 100 degrees celsius, where on fahrenheit is is 32 and 212 degrees fahrenheit. Another, lesser used scale, is the kelvin scale.
Radiation, conduction, and convection.
Radiation is is the direct transfer of energy by electromagnetic waves, most of the heat that you feel is infrared radiation. Conduction is the direct transfer of heat from one object that is touching another. When a faster moving molecule (the hotter molecule) bumps into a slower moving molecule (the cooler molecule) there is a transference of energy between them, this is how you and tell if a certain object is hot or cold. Convection is whenever the molecules move in a liquid or gas and their energy goes with them, causing a transfer of heat.
This is radiation, the sun is radiating heat onto the earth.
This is conduction, the heat source is transferring heat to the skillet.
This is convection, the water molecules are moving around transferring the heat around the water.