Basics of OFDM Technique in Communication Systems Explained

can somebody explain me the basics of OFDM technique in communication systems ?

I would start by understanding Frequency Division Multiplexing (FDM). The approach with FDM is to send multiple signals on different carriers (frequencies) at the same time. The spacing between these frequencies is far enough apart that there is no interference between the different channels.

OFDM is similar to FDM, but within the channel there are sub carriers. Spread spectrum is used to maximize efficiency and avoid interference between sub carriers. Check out the attached images.

Check out this "link":http://zone.ni.com/devzone/cda/tut/p/id/3740

Is it just me, or does anyone else feels the same.
When you have to describe the signal with a mathematical formula and can not make a descent picture.
Ten things get much harder to understand.
With Qamm or Qpsk u just write a picture and get a grasp of where the problem may be, its harder in OFDM. Thoose pictures I have seen so fare of OFDM does not give me any grasp of well anything....
Lucky me that I do not need to know
On the other hand I'm always that curios

I always find pictures help. Especially in calculus, once I could picture a problem the math came much easier.