It is all about slope!
Slope = Change in YChange in X

We can find an average slope between two points.
 
But how do we find the slope at a point?
There is nothing to measure!
 
But with derivatives we use a small difference ...
... then have it shrink towards zero.

Let us Find a Derivative!
To find the derivative of a function y = f(x) we use the slope formula:
Slope = Change in YChange in X = Î”yÎ”x
And (from diagram) we see that:
x changes from  x  to  x+Î”x  
y changes from  f(x)  to  f(x+Î”x) 
Now follow these steps:
 Fill in this slope formula: Î”yÎ”x = f(x+Î”x) − f(x)Î”x
 Simplify it as best we can
 Then make Î”x shrink towards zero.
Like this:
We write dx instead of "Î”x heads towards 0", so "the derivative of" is commonly written
x^{2} = 2x
"The derivative of x^{2} equals 2x"
or simply "d dx of x^{2} equals 2x"
"The derivative of x^{2} equals 2x"
or simply "d dx of x^{2} equals 2x"
What does x^{2} = 2x mean?
It means that, for the function x^{2}, the slope or "rate of change" at any point is 2x.
So when x=2 the slope is 2x = 4, as shown here:
Or when x=5 the slope is 2x = 10, and so on.
Note: sometimes f’(x) is also used for "the derivative of":
f’(x) = 2x
"The derivative of f(x) equals 2x"
or simply "fdash of x equals 2x"
"The derivative of f(x) equals 2x"
or simply "fdash of x equals 2x"
Let's try another example.
Have a play with it using the Derivative Plotter.
Derivatives of Other Functions
We can use the same method to work out derivatives of other functions (like sine, cosine, logarithms, etc).
But in practice the usual way to find derivatives is to use:
Using the rules can be tricky!
So that is your next step: learn how to use the rules.
Notation
"Shrink towards zero" is actually written as a limit like this:
"The derivative of f equals the limit as Î”x goes to zero of f(x+Î”x)  f(x) over Î”x"
Or sometimes the derivative is written like this (explained on Derivatives as dy/dx):
The process of finding a derivative is called "differentiation".
You do differentiation ... to get a derivative.
Where to Next?
Go and learn how to find derivatives using Derivative Rules, and get plenty of practice.