"Creation" or "Creationism" is based on the revelation from God as found in the first part of the book of Genesis in the Christian Bible Old Testament (or the Jewish "Torah"). The creation is described as taking place over a six-day period, after which God rested (and therefore no more creating has taken place since). Strict creationists believe that these were 24-hour earth days and that the earth is quite young, while others calling themselves creationists take these to be "long periods of time" (millions of years) and typically believe the earth to be very old (around 4.6 billion years as is commonly taught). The arguments in both camps are extensive. Some of the main points are:
In favor of 24-hour days: the text seems to go out of its way to make sure each "day" is understood as a normal day through the use of the words "evening" and "morning" associated with each day; unless there are compelling reasons the text should be interpreted naturally (as actual days); given that God was doing the creating, the real question is "why did He take so long?". Also, as we know from the theory of relativity, it is possible that from the frame of reference of the earth six 24-hour days DID take place, while a much longer period of time may have elapsed in the rest of the universe (which could account for the time for distant starlight to reach earth). Also, the Bible says death entered the world with the fall of man, implying there was no evolutionary animal death before the appearance of man on the earth.
In favor of long periods of time: a "day" CAN mean an indefinite period of time, and does mean this in some places in scripture; there is a lot of activity on day six (animals and Adam created, Adam lonely, Eve created, animals named); some scientific evidence seems to indicate the earth is old (such as radiometric dating of some rocks).