If it's a relay board, they are often set up with optoisolators - you can sink current (low signal) to turn it on, or it can be wired so that you source current (high signal) to turn it on.
Most that I've seen recently need a low to turn them on.
NC vs NO and COMMON - relays can be used to connect 1 of 2 signals to input for example, or to switch an output to 1 of two inputs - or just as a power switch as you are doing. So three different uses right there - need to think broadly about these things.