A tower is a tall structure, taller than it is wide, often by a significant factor. Towers are distinguished from masts by their lack of guy-wires and are therefore, along with tall buildings, self-supporting structures.
Tokyo Skytree, the tallest tower in the world
Azadi Tower in Tehran, Iran; an example of Iranian architecture of various periods
Roman tower (reconstruction) at Limes – Taunus / Germany
Eiffel Tower in Paris
A guy-wire, guy-line, guy-rope, down guy, or stay, also called simply a guy, is a tensioned cable designed to add stability to a freestanding structure. They are used commonly for ship masts, radio masts, wind turbines, utility poles, and tents. A thin vertical mast supported by guy wires is called a guyed mast. Structures that support antennas are frequently of a lattice construction and are called "towers". One end of the guy is attached to the structure, and the other is anchored to the ground at some distance from the mast or tower base. The tension in the diagonal guy-wire, combined with the compression and buckling strength of the structure, allows the structure to withstand lateral loads such as wind or the weight of cantilevered structures. They are installed radially, usually at equal angles about the structure, in trios and quads. As the tower leans a bit due to the wind force, the increased guy tension is resolved into a compression force in the tower or mast and a lateral force that resists the wind load. For example, antenna masts are often held up by three guy-wires at 120° angles. Structures with predictable lateral loads, such as electrical utility poles, may require only a single guy-wire to offset the lateral pull of the electrical wires, at a spot where the wires change direction.
Guy (red arrow), controlling the spinnaker pole.
Sidewalk guy with yellow guard, used due to the limited space between the pole and railing to the right
Closeup of anchor end of three guy-wires used to support the KVLY-TV mast in North Dakota, the tallest guyed mast in the world. Each guy is one member of a set of three that is located radially around the tower
Guy-wire supported mast.