This is a really good site that discusses the two topics. Follow the links at the bottom to find the corresponding article on Infallibility.
In short summary inerrant means "without error" and infallible means "incapable of error. The reason why those seem very similar is that, with respect to the Bible, they are. The word infallible is normally applied to the church, and is a key part of Roman Catholic theology. It means that the church has not only promulgated no errors in the past, but will not do so in the future.
Protestants (who mostly do not recognize the infallibility of the church) have tended to apply the word to the Bible, which they take as their source of ultimate authority. However the word is not nearly as applicable to the Bible, as the Bible is a closed canon, and cannot make any statements "in the future".
The Wikipedia article on Biblical infallibility quotes a theological as saying that 'infallibility' is used by some theologians to refer to the doctrine that the Bible is correct in matters of faith and practice only.
So in short, when applied to the Bible, the two words are virtually synonymous. When applied to other authorities, like the church, they can mean very different things.