All these terms refer to the new RGB LEDs that people are using. They are individually controllable at the single light, or Pixel. These are available from other sources, but RJ has developed his own controller to control them which can make it much cheaper than buying a premade system. He called them Smartstrings and info can be found in the wiki here You are not allowed to view links.
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LoginLet me tell a little about the terms you specifically mentions.
Flex Strip and RGB strip would both refer to a strip of leds on a plastic backing that is flexible. These are the ones that work with RJs controlloer. You are not allowed to view links.
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Login. Each LED out of the 120 can be controlled. It has a wrapping around it that does kinda make it look like a rope light, but again, you can change the color of each LED
Rectangular Node refers to these. You are not allowed to view links.
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Login. These are a little like the flexible strip, but cut into groups of 3, then connected with wire. Each group of 3 will be the controlled the same. The Square node is the same, but the LEDs are aligned on a square backing. Like this You are not allowed to view links.
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LoginThe Pixel string refers to these. You are not allowed to view links.
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Login. These would most look like what you would get from Home Depot, but again, each one can be individually controlled.
There are different uses people are using them for, look at some of the videos people have posted to see what you can do with them.