You start off in a little boat, with few resources. You are both a soldier and a pioneer. Your task is to land on this new world, explore, and colonize. The interface is similar to Civilization.
Colonization is a very big game with loads of options.
The replay value approaches infinity and it is very complicated. Good online help is available and the interface is intuitive, but I recommend a good read of the manual. I found them very useful. Although the manual is written for the PC version, it doesn't really matter. The Mac-specific instructions come on one of those annoying pieces of paper that developers seem to put in as a last minute extra.