Circularity involves an argument chasing its own tail. In circular arguments, the arguer assumes as a premise the conclusion that the arguer intends to draw.: claim that A → B, B, so A.
Trimming the fat off many Christian arguments for God's existence:
The Bible is the Word of God (because the Bible says so)
Therefore, the Bible proves that God exists.
The Bible is the Word of God
The Bible tells us that God created man in His own image,
Therefore, God must have created man.
Therefore, God must exist.
(Descartes' ontological argument is frillier, but equally circular.)
A typical circular YEC argument might run:
Scriptures tell us that God created the world in six days and also state (by Bishop Usher's calculation) that the Earth is a maximum of six thousand years old
(We deny scientific dates and claim that) the Earth is very young
Therefore, God created the world
(Even if the Earth really were very young (it isn't) it would not necessarily follow that God created the Earth because some other cause might have operated. In addition to being circular, the argument above also creates a false dichotomy.)
Of course, debaters embed their faulty reasoning within layers of padding, so it may be difficult to discern the conclusion that is passing itself off as a premise. Be wary, and look more carefully at an argument whenever you suspect that the argument has not adequately explained how its conclusion could be justified by the facts provided.
Because their arguments are very weak, and because their claims are not supported by empirical evidence, creationists resort to desperate measures when engaging in debate. A common creationist ploy is to accuse an opponent of circularity when he or she has provided a scientific explanation. An explanation cannot commit the fallacy of circularity because an explanation is not an argument. In fact, explanations ought to be internally consistent and none of the components of an explanation is a premise or a conclusion.