So, it would be perfectly equal if I connect it to a -3v/-5v thing to a grd pin?

2 more questions:

1) What is default GPIO output? 3v or 5v? And how do I change from one to the other?

2) What are the pins with special labels? (TXD, UART, I2C)

Hello! I am a total begginer to both eletronics and raspberry pi, and I have 2 questions. First. Is there any difference between pluggin the LED circuit into the -3v pin and the grd pin? arent both of them the same? If not, what should I use the grd one for? Second, I built this basic circuit: http:...