An in and out is a two jump combo that can be a one or two strides. A bounce - there is no stride taken in between the two jumps and a line you have several strides between jumps. Hope that helps, Happy Jumping :)
An in-and-out to me is a 1 stride, set at about 24 ft for the average horse. However, a smaller strided horse (or pony) may only be able to do it in 2 small strides. I think generally I would still call this an in and out, as that is what it was meant to be ridden in. 2 strides would be... well, a "two stride". :) 3 strides and more to me is a line. When a horse lands and immediately takes off is a bounce.