The Question: Is concept art just an idea of something, or multiple ideas to choose? Or basically a sketch for a creation to show the idea of it. Cause i have seen concept arts in many form, multiple same sketch of different design, sketch of one thing that looks quite like an finished art, messy sketches of shapes and form of an entire movement or action. I am very confused. But my main interest is to create something for a game/movie > character, costume, weapons and props. Is concept art really the right path? I know that concept art is the very start of the process in a creation of a game/movie. Sorry for so many questions and long reply...
My Answer: The answer to all those question is only one though: Concept Art is a process, usually driven by an artistic media, used to concretize an idea in order to create a final product (potentially everything that can be made).
So potentially concept art can be made for a lot of reasons and with a lot of purposes but there are common ways to do it. I think the most common ways today are the silhouette and the happy accident sketching, together with gestural drawing and lasso. That's the reason why everytime we talk about concept design itself the techniques and the look of it could be vary but the meaning it's always to generate an idea or to fully render a design for a product.
I hope this will clarify most of your doubts, in your case when drawing an armor you must first think "what's this armor for? is this for a movie, a videogame or a comic ecc? what's the purpose of this armor? how this armor will work?" and all that kind of stuff.