Each individual element of a pixel string is a group of LEDs (usually red, green, and blue) along with a chip (an integrated circuit, or IC) to control them.  Each chip has an identifying part number.  The most popular pixel chip is WS2812.  Others are WS2811, APA102, and APA104.  New chip types are being developed all the time.  The RC4 DMXpix and LumenDimPix can support new chips by updating firmware.