A skyline is the outline or shape viewed near the horizon. It can be created by a city's overall structure, or by human intervention in a rural setting, or in nature that is formed where the sky meets buildings or the land.
Skyline of Lower Manhattan. The term "Skyline" was first used for New York City in 1896.
Skyline of Tokyo
San Gimignano Towers in Tuscany, Italy
Mount Everest
The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whether it intersects the relevant body's surface or not.
The curvature of the horizon is easily seen in this 2008 photograph, taken from a Space Shuttle at an altitude of 226 km (140 mi).
View of the ocean with two ships: one in the foreground and one to the left of it on the horizon
A view across a 20-km-wide bay in the coast of Spain. Note the curvature of the Earth hiding the base of the buildings on the far shore.
Typical desert horizon