It's inbreeding when it doesn't work out well, lol.
Seriously though, inbreeding IMO is when a horse has the same sire or dam 2 or more time within 3 generations, or 5 or more times within 4 generation.
Linebreeding is when you have two similarly bred horses top and bottom, like two halter horses that both go back to Skipper W once or twice through different sons/daughters of Skipper W.
Inbred horse (I owned this one, never bred her): Dark Dancer Paint
She goes back to Skipper W 11 times in 7 generations. OUCH!
Another inbred horse, same kind of lines: Skips Ego Improver
Has TWELVE (12) crosses to Skipper W in 7 generations
Has THIRTY (30) crosses to Nick (24 through Skipper W and 6 more from others)
He was bred to this mare: Shawnee Sandy Quarter Horse
Has three (s) Crosses to Skipper W in 5 generations
Has THIRTEEN (13) crosses to Nick in 7 generations (6 from Skipper W, 7 from others)
So the resulting filly/mare has SEVENTEEN (17) crosses to Skipper W in 8 generations and FOURTY-THREE (43) crosses to Nick... Whoa. This mare only has 4 unrelated lines/horses out of 16 (at the great-great line)... Both of her parents were heavily related. That means 12 of her great-great relatives were very closely related. Scary indeed. It would make a great Springer show, lol.
The first mare listed has a Sire that IMO is line bred: Skipa Sonny Paint
He has Skipper W once on top and three times on the bottom, far enough back to not be inbred IMO. There are also some related lines to Skipper W, though pretty far back.