In my head I think of it like this:
L.E.D = Light Emitting Diode ( single light source, kind of like a silicon light bulb, 2 wires)
R.G.B = Red Green Blue ( three colours )
R.G.B. L.E.D = A LED made up of three other LEDs, one each of red green & blue, 4 wires, one of each colour and a common)
Channel = A software controlled switch which can only turn on and off a single LED or colour in a RGB LED. Hence an RGB LED requires three channels to be controlled.
Pixel / Node = A single controllable point of light. Takes three channels to allow any colour to be created. Just like the dots in your TV or computer screen. It can made up of three LEDs one for each colour or a single RGB LED. Some pixel are made of multiple RGB LEDs for extra brightness. There might be any number of LEDs but the whole thing is still a single pixel or node.
Pixel strings are a string of small pixels/nodes typically made of 1 RGB led per pixel/node. They come in various lengths. You can cut and splice them at will, provided that you do not exceed 128 pixels/nodes and you keep the max length between nodes down to around three feet.
Square and Rectangle Pixel nodes are just that. A pixel in a square shape or a pixel in a rectangle shape. You can connect up as many or as little as you like, and you can even mix them up with the smart strings, mix and match to suit your display, the same limitations apply as the smart strings.
Flexible strip is a flexible bendable strip of 120 pixels/nodes. I believe you can cut to shorter lengths of a fixed size. It can be treated the same as the pixels/ strings above so you could have a 1 meter (3 foot) length of flexible strip (30 pixels) with 5 square pixels/nodes stuck on the end.
Rigid strip is a solid strip of 30 pixels/nodes. I dont think you can cut it, but you can connect them up to other pixels/nodes keeping in mind the above limitations.
A smart string controller (SSC) is a device which converts pixelnet data into pixel/node language. It can control a maximum of 128 pixels. It also has a limit to the amount of current or power that it can put out, so the number of devices is also limited by that. (128 pixel string nodes, 120 rectangle, 80 square, 4 rigid strips or 120 flexible strip nodes - or any other combination that uses the same amount of power. )
The smallest element you can control is a channel, but that is not that useful because you need three of them to do anything. The smallest element you can physically get is a pixel/node. The term element is confusing because it can apply to both channel or pixel, which are clearly different because a pixel needs 3 channels (RGB).