This is one of the most contested and confusing areas of genetics. Linebreeding is a commonly used term but very few agree on what that actual means. Inbreeding is another term which is different depending on who you speak to.
Linebreeding is inbreeding but inbreeding is not neccassarily linebreeding.
Linbreeding is the practice of tyring to extract large quantities of genetic material from ONE fish...the practice of how this is done varies and is not universally agreed upon but generally speaking means that a male fish will be mated to it's daughter, then to the offspring of that mating and so on. The idea being that the offspring will retain as much of the good qualities of the chosen fish as possible...this is a high risk strategy though because you're increasing the chances of bringing reccessive genes to the surface by allowing two copies of a defective gene to show in a single fish...if a fish only has one copy it will not normally create a problem. Although selecting from good stock helps reduce the chances of this there is no such thing as perfect genes and even great fish will carry some bad ones.
If you use a female fish some people call it family breeding, effectively though it's the same thing.
Inbreeding is more chaotic. It happens every time you mate two fish that share genetic material...the level or percentage of inbreeding is what's important to whether genetic faults will show. Too high a percentage and the risk of two faulty genes cropping up in one fish increases in the same way as linebreeding.
Linebreeding and inbreeding are both used by breeders to 'set' characteristics into their fish, it is a neccassary and normal pratice.
How much is too much? That depends on the fish selected and the percentage of inbreeding. the only way you can really tell that you have inbred too much is when you start getting faults, you then need to introduce fresh blood to eradicate them. Generally speaking one generation of inbreeding carries little risk.
Sorry for rambling