Questions regarding Mirage

Now, before anyone ask me to google, I've already did and I'm still a bit confuse on how mirage happen.

Before I start my 'research', the few things that I know about mirage is; cold air, hot air and light.

After googling and wikipedia-ing, I came across quite a number of things. Temperature gradient, light particle traveling faster through hot air and hot air being less dense. Anyway, I'm not entirely sure how to even ask... so,

Can anyone explain to me with simplicity but detailed way? Also include;
- What exactly is temperature gradient and what causes it.
- How exactly are the movement of the hot/cold air and lights goes.
- What are the differences in term of movement of stated above results in inferior, superior and fata morgana.
Overall, my questions is, how exactly do mirage occurs?

Thanks in advance!

Basically there are two masses of air that are different temperatures. one is on top of the other. the junction of the two creates a very imperfect mirror pointing back down towards the earth, this is because of the difference in density of the two air masses.
you get the same effect in a swimming pool under water when you look up at the surface at an angle.
a Mirage and a fata morgana are not the same thing.