Once again, an I/O card (input/output) is a device that you can connect a bunch of wires to. When you touch two wires together, or apply a voltage to them, it will trigger something to happen on your computer. For instance, when you push the power button on your television set, it turns the power on and off.
Digital Signage is more of a Digital Sign. Nothing to do with sgnatures. Typically a television set located in a store advertising a product or promotion, in place of standard cardboard or plastic signs that do nothing else but display one static image or logo. A digital sign can be changed easier than a traditional sign, and often remotely from the internet. Digital signs can gather information from the internet and display it on the screen, such as weather forecasts or stock info. Digital signs are located everywhere from airports, to restaurants, to amusement parks.
If you STILL don't understand, then forget that term altogether, I just want to play a video by pushing a button, external from the computer.
Are you familiar with X-10?
That would be an example of I/O
Here are some pictures of Digital Signs (Digital Signage is the technology)