The most common tort--and the one most difficult to define--is negligence. Negligence is defined as the failure to use reasonable due care to avoid a foreseeable harm to a person, place or thing. If you are negligent and your negligence causes injury to another person to whom you have a "duty of care," you may be liable to pay any damages resulting from the injury caused by your carelessness.
A person may be considered "careless" or "negligent" if they do not use the kind of "due care" that is appropriate to the particular situation in question. For example, a higher level of care is called for if you are pouring boiling coffee into a friend's glass over his lap than is called for if you are pouring cold lemonade over the kitchen sink. Generally, the law requires that individuals exercise the same kind of "due care" that a reasonable person would exercise under the same circumstances. This is called the "reasonable man" or "reasonable person" standard.