Sergio, this might help:
Normally when we are controlling lights we use one CHANNEL to turn a string of lights on, off or Dim somewhere in between those two points. In the case of a Lynx Express it has 16 channels so you could control 16 separate strings of lights with one Lynx Express.
Now with Pixels it is much different. With a Pixel String each "bulb" is a node consisting of 3 LED's Red, Green and Blue. So each node is 3 channels and if you have 50 nodes on a Pixel string you would need 150 channels to control that string. By setting a dimming value for the Red, Green and Blue LED's in a node you can create a specific color for that node.
Universe- X number of channels depending on the universe type (512 for DMX OR 4096 for Pixelnet)
Channel - single part of a universe
Node - 3 channels (Red, Green and Blue) single element of a Pixel string
Hope that doesn't confuse you to much.....