The definition of a pack rat depends largely on one's point of view. In the wild, it is a small rodent that lives communally, in large nests called middens, and collects any found objects to assist in building the nest. Among humans, this term is used for a person who keeps everything and has a hard time throwing anything away.
Most people who consider themselves to be pack rats hate to throw things away because of the fear that they might be needed at some point in the future. This type of behavior can be practiced to greater or lesser extremes, however; one person might have a difficult time throwing things away but still be able to maintain a clean and well organized home, while another might quickly lose control. Hording may be a sign of mental illness and can cause serious social and health-related problems.
The main problem for a pack rat is what to do with all the stuff he or she accumulates. Organized individuals usually find space to store their stuff in one place and may be able to keep the amount of stuff that they have to a controlled level. Hoarders and other disorganized people usually stores the stuff all over the house — any flat, level surface becomes storage, whether it is the dining room table or an ironing board.
When hoarding behavior gets out of control, it can have serious consequences. Friends and family may avoid the person's home, since it's filled with items; the individual may even be reluctant to invite people in for fear that they may discover the seriousness of the problem. Eventually, sheer amount of stuff can allow it to become a breeding ground for insects and rodents, and mold and mildew might grow on buried items that have become damp. Individuals who keep animals often cannot care for them, leading to them never being cleaned or their waste disposed of.
The term "pack rat" often has unflattering connotations, and some people prefer names like "collector" to avoid the hoarding stigma. Ideally, this person should strive to keep only those things that are truly important, throw away the detritus, and maybe have a yard sale once in a while. Some community education programs offer one-day classes on organizing closets and homes, and these may be helpful if the clutter level is getting unmanageable.