What's NO and NC in switching devices?
Suppose you purchase an electronic switch, which turns on when you apply 12 volts to its designated pins. And turns off when you remove the voltage.
If that's the case it will be called Normally Open device, "NO". Stays "off" in normal circumstances. Without a voltage.
The other type of device is called Normally Closed. NC. Stays on normally. And turns off when you apply voltage.
There's use for both. And both are used in equal frequency in switching.
Summary :
NO : normally off. Mnemonic NO = NO current.
NC : normally on. Mnemonic NC = Normal Current.