NASA, and our Military refer to manned trips to space as “Missions” and locations for landings as “bases”. We are beginning to see space advocates talk of those same landings as “colonies”. And more rarely, a small minority are looking forward to “Settlements”. In many circles, these four terms are used and abused interchangeably seemingly without regard to any understanding of their meaning, or history. Let’s look at them.
First, The Mission …
This most often means performing tasks like a script, to meet specific pre-defined goals. A mission is often characterized by a time-frame, with start and stop times. The mission plan is followed specifically until complete with limited variance. A mission has a beginning, middle, and … End.
Having served in the military, I am familiar with this usage of mission, and the related term “base” as in “base of operations”. A base, as used by space advocates and NASA alike aludes to a facility placed in space or on the Moon or Mars where “Mission Tasks” are executed to achieve “Mission Goals”. The ISS represents a “Base” of operations for science operated by representatives of Earth. “Mission” goals are accomplished by the team members each day the station orbits the Earth.
Next Colonies …
A “Colony” is an area that is 
controlled by or belongs to a country and is usually far away from that country. Conversations online and in the news often reference this term when discussing humans leaving Earth to live on another planet, or in space. But it is important to remember the historical context of this term as well as this definition. Colonies are subject to the country-of-origin – always. The colony cannot be self-directed, in fact, by the designated charter, colonies have been established for the betterment of the “mother-country”. The people, equipment, and tasks are hand-picked to accomplish the specific goals of the “Mission Plan”. Even though a colony assumes multiple generations of humans occupying it, those generations are all subject to the Mission Plan, and the mother-country … whether they like it or not. Interestingly, Colonies are almost never self-supporting, but nearly always dependent on supplies shipped form the mother-country. Many times this isn’t so detrimental, but historically … it’s always becomes a bad thing.
Now, Settlement …
Miriam-Webster defines Settlement as “the act or process of settling”. Further, the definition of “Settle” is listed as: “to move to a place and make it your home“.
To Summarize:
A Mission is a targeted task, or group of tasks performed as assigned by superiors.
A Colony is a location populated by people to further the goals of a country … regardless how far away that country is.
A settlement is a home, established by free individuals.