My Pi 2 kit came with the following GPIO pin reference card.
In addition to showing all of the 5V, 3.3V, GND, and GPIO pin numbers, it has additional information on some of the GPIO pins like TXD, RXD, UART, CLK, PCM, DE0, CE1, SPI, DIN, DOUT, PCM, SDA, SCL, I2C, MOSI, MISO, SCLK, SPI, PCM, FS along with various colors on these.
What does all of this mean? I am assuming these are special pins for something. Are not all of the GPIOs usable in the same way?