Someone who's wasteful uses and disposes of things carelessly. A wasteful child might eat two bites of yogurt and then throw the rest of the container away.
You can be wasteful with your money, spending too much of it on unimportant things. Some people are wasteful with food, ordering too much at a restaurant and leaving most of it on the plate. It's possible to be wasteful with almost anything that you squander without thinking about, including electricity or heat or even your own creative energy. The Latin root of waste is vastus, "empty or desolate."