The most Common Pixel is the Bullet style that is shaped like a bullet and the square style is shaped like a square. The most common pixel type is the WS2811 which almost ALL controllers can control.
So when you are ordering you most likely will be ordering some like this:
12V WS2811 / IP68 / Bullet Pixels / 18 gauge Black wire / 4 inch spacing / 100 Count / with Pigtails
Which Pigtail? Continue onto the next section in lights: Connections