Heat is the amount of thermal energy getting transfered(didn't expect that, hu?
). "So what is thermal energy, then, smart***?" - you may ask 
Thermal energy is the amount internal kinetic energy(And kinetic energy is motion/movement/whatever...
). Internal kinetic energy is the kinetic energy of the molecules and atoms random movement.Now we know that thermal energy is motion. So how does this energy transfer?
There are three ways we know:
Conduction
This is when the particles are in physical contact. When a particle collides with another one, it transfers part of its own kinetic energy to the other particle. This kind of transfer can only happen in physical contact. You can try this by rubbing your hands together for a few seconds, same thing happens. It doesn't matter if the object is solid, liquid or gas.
Radiation
This is another transitional form of thermal energy. The radiation is created by electrons moving to upper energy levels and coming back closer to protons losing energy resulting in waves(radiation). Radiation can travel easily through space, even through a vacuum. It's called infrared radiation BTW, it travels at the speed of light. Nuclear reactions can cause radiation too, but it'll be much higher. You can calculate how much energy(not only thermal energy) nuclear reactions create by this: mass * speed of light˛. I'll talk about this more on my next topic about fusion, fission and fire. Anyway, when something stops radiation(like a brick wall), it heats up(correctly: gains more thermal energy.) The object can also reflect energy. Living things are viewable in infrared cameras because they generate infrared radiation by chemical reactions in mitocondria (it's the part of the cell that burns material).
Convection
This one isn't very important. To have thermal energy to transfer by convection, the matter has to be in liquid or gas form as flow is required for this. Simply, the object has thermal energy, it's moves to another location and mixes with other "stuff" with higher or lower amount of thermal energy. After the objects mix and become one(now conduction starts working), they have an average amount of thermal energy somewhere between the original ones. One example of this is air conditioning.
There are upper and lower limits for thermal energy.
Upper limit
It's the speed of light. The particles can't move faster than the speed of light, they can't even reach the speed of light. It's the upper limit for any form of energy if it relates with speed and mass.
Lower limit
This is absolute zero. It's 0 Kelvin(Kelvin is the standard scientific unit to measure temperature and/or thermal energy). There can be no less internal kinetic energy than no internal kinetic energy. BUT, absolute zero can't be achieved because a) there will always be something giving the object more internal kinetic energy even if it's a closed system.
the laws of quantum mechanics(too hard to explain right now what it means... google is your friend). By the laws of quantum mechanics I mean that if particle doesn't have any kinetic energy, it doesn't exist.Solid, liquid and gas
These are the forms of matter, and they depend on the amount of thermal energy. If you compress a lot of gas, you get a lot of heat. As gas needs heat to remain as gas, if you compress it, it'll turn into liquid. So all the internal kinetic energy has to go somewhere right? The same thing happens in your fridge, liquid is turned into gas, and gas absorbs the "heat", cooling your beer
The unit we measure heat with is called a calorie. 1 calorie is the amount of heat that needs to be transfered to 100 grams of water to raise it's temperature by 1 celsius or Kelvin. Thermal energy itself is measured by Joule (J) like any other form of energy.
Thermal energy also means the potential energy of the object. The internal kinetic energy plus the potential energy form thermal energy. I mentioned this later so I wouldn't confuse you, but confuse you at the end
I hope I haven't confused you much. So now you know, happy now?
