I would like to start this section by telling you that inbreeding is not a common practice at Whispering Woods Kennel. Technically speaking, all purebred dogs are linebred to some extent. The following should explain these terms more precisely.
From a scientific point of view, line breeding is defined as breeding two individuals containing at least one common ancestor. This common ancestor may contribute a negligible amount to its descendants, or a great amount. For example, let's say that 200 years ago a dog existed that sired a litter of 2 pups. One pup was flown across the ocean and one stayed here. Both were bred. Now, 200 years later, the descendants of both trace back to the father (and mother) of both. If two of these dogs were bred, the genetic influence of that original stud dog would be minimal, but it would still be line breeding.
Close line breeding is inbreeding. Breeding parent to offspring is close line breeding, or inbreeding (see below).
Inbreeding is generally defined as the breeding of two closely related individuals. Technically speaking, however, it can be defined more precisely.
Consider the fact that any sexually reproducing life form obtains 50% of its genetic material from each parent, 25% from each grandparent, 12.5% from each great grandparent, etc. Scientifically speaking, an inbred animal contains more than 50% of its genetic material from one individual. For example, breeding a parent to its own offspring will produce animals containing over 50% of their genetic material from the animal that is the parent and the grandparent both. Breeding a brother to a sister will produce offspring with up to 50% of their genetic material from one grandparent and at least 50% from the other they will only have 2 grandparents. It is so unlikely as to not even be worth considering that they would get exactly 50% from each, but if they did, they would be full-siblings to their own parents and they would not technically be inbred, but there would be no way to tell.
Inbreeding can be catastrophic or it can be the absolutely best way to improve and standardize a line. Any hidden, recessive traits in the ancestor providing the majority of the offsprings genetic material are reasonably likely to show up. These traits can be good or bad. Oftentimes, "test-breedings" are done to verify the superiority of a particular strain or to check for recessive problems. Any weaknesses, faults, deformities, etc. are likely to show up. If a truly superior line or strain has been developed, containing only desirable qualities, these desirable qualities will be seen in the resulting offspring.
There are 2 specific (and 1 general) examples I can cite; The white mice used in labs, called "Jaxon mice," have been inbred parent to offspring and brother to sister for over 1000 generations. At this point, they are so identical as to be virtual clones of each other. The only difference to be found is that there are both males and females. This is why they are valuable for research; there are less variables in any experiment as all the "test subjects" are genetically identical, and should respond alike to any stimuli. It is also a fact that these mice are healthy albinos.
Guiding Eyes For The Blind, Inc., located in San Raphael, Ca., has developed 3 strains of German Shepherd Dogs to guide the blind. One of these strains, the "Frankie line" is being inbred to produce offspring containing as much of Frankies genetic material as possible, some having as high as 96% of their genetic material from Frankie. And these dogs are happy, healthy, well-adjusted and successfully guiding their blind owners through city traffic, etc. (Another strain is geared to line breeding to produce puppies that have as close to exactly 50% of Frankies genes as possible line breeding. The third strain is based on another dog [Orthos], and they are trying to increase the percentage of his genes in the puppies to the highest possible extent, while still maintaining something genetically from Frankie.)
Also, many farmers/stock-breeders will buy a superior bull, buck, ram or stallion and breed his daughters back to him. Their female stock is improved in this way, and after a generation or two (or three?), the stud is sold, leaving behind a much more valuable female breeding population, and then a new superior stud is found, and the process restarted.
It is important to note, however, that in the pet market, most inbred litters occur because of the wrong reasons. Accidental breedings for example, or a male pup didn't get sold and it was a cheap way for mom to have another litter, etc. Inbreeding should only be attempted by knowledgeable, experienced breeders, and even then, some unexpected surprises may be lurking!
In summary, inbreeding occurs when offspring are produced containing more than 50% of their genetic material from one individual; The totality of qualities belonging to that individual AND the knowledge of the breeder will determine whether this is a horrible thing or a wonderful thing.