RGB Pixels are digitally-controllable lights you can set to any color, or animate. Each RGB LED and controller chip is molded into a 'dot' of silicone. The dots are weatherproof and rugged. There are four flanges molded in so that you can 'push' them into a 12mm drill hole in any material up to 1.5mm/0.06" thick. We also have pixels that are flat, both are the same electronics, just different shapes!
They're typically used to make outdoor signs. We also have flat-backed pixels that are essentially the same, but are not as long and thin. The pixels are connected by a 4-conductor cable. +5V (Red), Ground (Blue), Data (Yellow) and Clock (Green). Data is shifted down from one pixel to the next so that you can easily cut the strand or attach more onto the end.
Each dot is digitally controlled, with an internal 8-bit PWM LED driver (24-bit color for 16 million different shades). The pixels must be clocked by a microcontroller, we have an example code that works on an Arduino. Just adapt it if you are using another microcontroller.
The pixels use 8mm diffused RGB LEDs, with a 120 degree beam width. The total max brightness of all LEDs is about 1600mcd but with the light more evenly distributed & mixed than a clear LED. (Please note: mcd ratings of LEDs are notoriously inflated by most LED sellers, so be extra-skeptical when reviewing LED ratings!)
Sold by the strand, each strand has 25 pixels in series! Each strand has two JST SM 4-pin connectors so you can connect multiple strands in a row, as many as you wish, just watch for how much current they want. We now have LED pixel strands with the power wires (red & blue) spliced out so its really easy to connect 5VDC in using a 2.1mm jack terminal adapter.
We have a 5V/2A supply that drives 2 or more strands and a 5V/10A supply that can drive up to 160 LEDs (6 strands) all lit up at once! If you want to connect to the input of a strand (to wire it to power and a microcontroller) please get a receptacle connector. If you want to connect to the output, get a plug cable. You can drive these with an Arduino using any two microcontroller digital pins.
Check this library that also has example code for demonstrating the strands and be sure to read Adafruit's very detailed tutorial on usage!