A plane is a craft which starts horizontally and utilises the lift generated from wings or winglets. Because planes usually use jet engines they are more efficient than rockets but normally don't leave the atmosphere. On one side this is useful as it continues to generate lift with wings but on the other hand does the atmosphere induce drag at all times preventing an stable orbit.
Planes are build in the Space Plane Hangar and start from the runway. It is possible to build planes in the Vehicle Assembly Building but from there planes would start on the launch pad, which is to short to accelerate and generate enough lift to start there. There are various stock planes available in the Space Plane Hangar.
The currently fast plane moved at about 2340 m/s relative to the ground a bit short of Mach 7. At this speed it is possible to circumnavigate Kerbin in about 29 minutes at an altitude of 40 km. For comparison the speed required to orbit Kerbin at 70 km (outside the atmosphere) is only 2296 m/s.
Another hazard to planes are flame outs which occur when not enough intake air is available. Especially when there are engines generating thrust not through the center of mass. This could lead to a unstable flight, when only engines on one side fail and engines on the other side continue to generate thrust applying torque.
Another form of planes are gliders. Those don't use any fuel to propel them and simply exchange height with velocity. As there is no weather no updrafts exits so gliders can't ascent without losing (much) speed. It is also complicated to get a glider in the atmosphere. There are infinite gliders which abuse the physic simulation and can accelerate without using fuel.
- “The MachingBird Challenge!” in the forums.