Complex Plane
No, not that complex plane ...  
... this complex plane: 
A plane for complex numbers! 
(Also called an "Argand Diagram")
Real and Imaginary make Complex
A Complex Number is a combination of a Real Number and an Imaginary Number:
A Real Number is the type of number we use every day.
Examples: 12.38, ½, 0, −2000
When we square a Real Number we get a positive (or zero) result:
2^{2} = 2 × 2 = 4
1^{2} = 1 × 1 = 1
0^{2} = 0 × 0 = 0
What can we square to get −1?
?^{2} = −1
Squaring −1 does not work because multiplying negatives gives a positive: (−1) × (−1) = +1, and no other Real Number works either.
So it seems that mathematics is incomplete ...
... but we can fill the gap by imagining there is a number that, when multiplied by itself, gives −1
(call it i for imaginary):
i^{2} = −1
And together:
A Complex Number is a combination of a Real Number and an Imaginary Number
Examples: 3.6 + 4i, −0.02 + 1.2i, 25 − 0.3i, 0 + 2i
Putting a Complex on a Plane
You may be familiar with the number line:
But where do we put a complex number like 3+4i ?
Let's have the real number line go leftright as usual, and have the imaginary number line go upanddown:
We can then plot a complex number like 3 + 4i
:
It is placed


And here is 4  2i : It is placed

And that is the complex plane:
 complex because it is a combination of real and imaginary,
 plane because it is like a geometric plane (2 dimensional).
Whole New World
Now we can bring the idea of a plane (Cartesian coordinates, Polar coordinates, Vectors etc) to complex numbers, and open up a whole new world of numbers that are more complete and elegant, as you will see.
Complex Number as a Vector
We can think of a complex number as a vector.
This is a vector.
It has magnitude (length) and direction.
And here is the complex number 3 + 4i
as a Vector: 
You can add complex numbers as vectors, too:
To add the complex numbers 3 + 5i and 4 − 3i :
separately like this: (3 + 5i) + (4 − 3i) = 3 + 4 + (5 − 3)i = 7 + 2i 
Polar Form
Let's use 3 + 4i again:  
Here it is in polar form: 
So the complex number 3 + 4i can also be shown as distance (5) and angle (0.927 radians).
Let's see how to convert from one form to the other using Cartesian to Polar conversion:
Example: the number 3 + 4i
From 3 + 4i :
 r = √(x^{2} + y^{2}) = √(3^{2} + 4^{2}) = √25 = 5
 θ = tan^{1} (y/x) = tan^{1} (4/3) = 0.927 (to 3 decimals)
And we get distance (5) and angle (0.927 radians)
Back again:
 x = r × cos( θ ) = 5 × cos( 0.927 ) = 5 × 0.6002... = 3 (close enough)
 y = r × sin( θ ) = 5 × sin( 0.927 ) = 5 × 0.7998... = 4 (close enough)
And distance 5 and angle 0.927 becomes 3 and 4 again
In fact a common way to write a complex number in Polar form is
x + iy  =  r cos θ + i r sin θ 
=  r(cos θ + i sin θ) 
And "cos θ + i sin θ" is often shortened to "cis θ", so:
x + iy = r cis θ
cis is just shorthand for cos θ + i sin θ
So we can write:3 + 4i = 5 cis 0.927
In some subjects, like electronics, "cis" is used a lot!
Summary
 The complex plane is a plane with:
 real numbers running leftright and
 imaginary numbers running updown.
 To convert from Cartesian to Polar Form:
 r = √(x^{2} + y^{2})
 θ = tan^{1} ( y / x )
 To convert from Polar to Cartesian Form:
 x = r × cos( θ )
 y = r × sin( θ )
 Polar form r cos θ + i r sin θ is often shortened to r cis θ
Next ... learn about Complex Number Multiplication.