The following wikipedia article explains quite well, I think, creationism.
In the broadest sense, anyone who believes God created the world is a "creationist" - as such, huge numbers of scientists are creationists, too - people of faith - and see faith and science as partners in life - one God, one world, and truth - material or spiritual - is consistent, not contradictory.
The struggle, especially in America under the influence of fundamentalism, has been Literalist Creationism - which begins with a literal reading of Genesis (the article is really helpful on this point).
If this were merely a tempest in a teapot, or Christians squabbling with themselves, who would care?
But across the nation, fundamentalists have made a concerted effort, successful in some areas of the country, to take control of local (and state) school boards. Where that effort has failed, many fundamentalists have opted for home-schooling or private schools - their science curricula is rather strange … and so is their history curricula (I've seen and read some of it on line).
At this point, the issue: quality education, the reliability of science (because God created an ordered world that we can accurately see, measure and even understand), the partnership of faith and science, and the accurate and faithful reading of the Bible and its many literary forms - some are quite literal: Jesus dies on Calvary (no metaphor there!) and some are metaphoric (Jesus is the shepherd, the door, bread and water - all metaphors).
Earliest Christians had no trouble reading Genesis as metaphor - searching Genesis for the spiritual meaning. Only in America here, and in the last 150 years, has literal creationism taken hold, and become an educational and political issue (literal creationists tend to be reconstructionists) - that is, to make the United States a theocracy (based upon Old Testament law) - ultimately doing away with the Constitution, which is often called "a godless document," because there is no reference to god in it) - similar to fundamentalist Muslims who want to impose Sharia Law.
It's complicated, for sure, and a lot is at stake.
Anyway, thanks for asking, "What is a creationist?"
Hope the above makes some sense.
Kindness is always in season! And so is truth!