A space colony is an artificial environment designed for human habitation not on the surface of any planet. The primary requirements are: Energy, which in space is provided by the Sun. Atmosphere, which is contained in a pressurized structure. Gravity, provided by the centrifugal force of a rotating structure. Radiation shielding, provided by a fixed non rotating wall of mining slag placed outside the rotating colony structure. Food, hydroponics farms probably separate from residential and industrial areas. The primary point of a space colony is that everything in the environment is totally controllable. It is a Human made place to live. For further reading, two books; Colonies in Space, by T.A. Heppenheimer; Space Settlements, NASA SP-413. There will be three generations of space colonies. One, those designed by people who live on Earth. Two, those designed by people who live in Space. Three, those designed by people who were born and grew up in Space and have never been to Earth.
Eventually the people who were born and grew up in Space will build something which I call, the Grid, thousands upon thousands of space colonies suspended in a grid structure in an orbit half way between Mercury and Venus. That orbit was chosen to maximize the available solar energy. The Grid is 20 colonies wide and extends along an unimaginable portion of the orbit around the Sun. Not merely a two dimensional structure, agricultural hulls and solar energy plants extend inward towards the Sun. Industrial, manufacturing, chemical, and nuclear power plants extend outward from the grid structure. Transportation is provided along the grid structure itself in magnetically driven trains. Even though human communities are scattered throughout the solar system, the majority of humanity resides in the Grid, things are just more convient there. From any point in the grid a human being can travel physically or electronically to communicate with more other human beings faster and more easily than at any time in history.
Facing the Sun huge photovoltaic arrays provide the energy to make human life in Space possible. The inner agricultural hulls allow plants to be specifically developed to maximize food production, in a controled environment there are no pests or disease. Although large livestock operations are difficult, a wide varity of products can be produced. In the outer industrial areas, environments can be tailored to fit the products being produced. Massive amounts of automation will be common in these hazardous environments. Because the solar wind sweeps contaminants out of the solar system, industrial pollution is much less of a problem.
For travel over long distances in the solar system it is necessary to use an elliptical liner. These are actually small space colonies in elliptical orbits. Most of the planets and bodies of the solar system have circular orbits. Examples of elliptical orbits are the comets which spend most of their time in the outer solar system between swings through the inner solar system. An elliptical liner would have it's perigee on one circular orbit and it's apogee on another circular orbit, allowing it to serve as a means of transportation between the two orbits. Because of the massive radiation shield of the colony design, which makes travel safe, it is extremely difficult to change the orbit of a colony in an elliptical orbit, so once it is set into it's orbit it remains there. Because the orbits of the origin and destination points often do not align with the position of a particular elliptical liner, a portion of the life span of the liner is spent uninhabited. Several liners may be used to serve one route with only the correctly aligned one carrying passengers, crew, and cargo.