Creationism is the belief that Earth was created by God, and all life was created by God pretty much exactly as we see it today. They self-identify as a counterpoint to evolutionists (or "Darwinists").
Creationists know this is true, because it's explained in the Bible. The Bible is true. We know the Bible is true because it's the Word of God. We know it's the Word of God because it says so in the Bible.
In other news, Harry Potter actually happened because the characters in the books claim it did.
Creationism is (roughly) divided into Young Earth creationism and Old Earth creationism. OECs believe what most others believe about the age of the Earth, but think God started the whole thing off. YECs, on the other hand, believe that Earth was created in 4004 BC. OECs are often reasonable and well meaning despite being wrong. YECs reject biology, astronomy, physics, etc., and are impervious to logic or evidence.
Creationism should not be confused with Intelligent Design, because they're totally different things. Nope, no resemblance at all. Honest. They don't even know each other. I wuz talking with Intelligent Design the other day, and he wuz all "Creationism? Don't know the dude. Definitely not my twin brother."