If you are careless, you do not pay enough attention to what you are doing, and so you make mistakes, or cause harm or damage.

If you say that someone is careless of something such as their health or appearance, you mean that they do not seem to be concerned about it, or do nothing to keep it in a good condition.

If you describe someone's movements as careless, you mean that they are relaxed or confident, and do not seem to require much effort or thought.