Basically it is the failure to act reasonably. It is based upon what a "normal" reasonable person would do faced with the same situation. Doing something carelessly or failing to do something that a reasonable person would do is negligence. For instance, running a red light and causing an accident would be negligence. In order to show someone was negligent you must prove the had a legal "duty" to act a certain way and that they failed to do so. Most personal injury cases are based on negligence- although some are intentional or based upon product liability doctrines.