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Gravity, the attracting force between all matter, also attracts planets. Due to the changes in the orbits of the known planets in the early 1900s, it was assumed that there was one or more planets beyond Neptune, but not yet identified. This search for these led to the discovery of Pluto, and since then a few other significant objects have been found. These are still too small to explain the perturbations, though, and revised estimates of Neptune's mass showed that the problem was fictitious.
The hypothetical companion star Nemesis would have fallen under the definition of trans-Neptunian object, but it appears that no such star actually exists.
Some kinds of TNOs: Plutinos, cubewanos, scattered disk objects.
Transneptunian objects of note:
See also: Oort cloud, Kuiper belt