Semiology is the study of making signals and signs, yet some people confuse it with linguistics. Linguistics is the is the study of language and speaking itself, but a good sign does not need a language to be effective.
When you are on the road, you see certain signs that signal what to do. Maybe you will see a red sign that says stop, or a yellow sign that says yield, or another sign for making a turn. Despite these signs all having language on them, the signs themselves have become a landmark themselves. This means that the color, shape and icons on these signs have become almost universal in communicating the same message to those who see them. If something is red, you know to stop, if yellow, slow down and a bent arrow is turn. The words almost become a second thought since we already know what to do from the signals made from the sign. The idea of semiology is to create this effect, a symbol so iconic, yet easy enough for people all around the world to understand without language barriers in the way.
Linguistics find less of a place on signs, but more of a place on graphic design and poster making. Being the study of language, it’s important to know the language of the audience you want to market to. You could have a signal that lures the person in, but most of your ads will be most effective based on the way they are written. If your ad looks interesting, but is poorly written, the consumer has just wasted their time and becomes a distraction. This is similar to signs as a bad sign distracts people and confuses people into doing misinterpreted responses from the sign. Overall, it is important to know that making a good sign is one that is devoid of the forces that bound us by the common language, that have the ability to invoke an action from a mere glance.