"Church fathers" is a loosely-defined term which is used to refer to early Christian theologians whose writings and teachings are considered by some to be authoritative in defining and defending Christian theology and practices. Much reverence is given to them in the Roman Catholic and Eastern Orthodox traditions, while Protestant sects tend to not place as much reverence on them in favor of sola scriptura (the Bible as the final authority). Generally those in the evangelical camp will consider them at some level of authority but not the final one, while fundamentalist camps tend to ignore them altogether due to their opinion that Catholicism is false teaching.
While there are fathers as late as the 8th century, church fathers are often divided into "Ante-Nicene"(prior to the Council of Nicaea, 325 A.D.) and "Post-Nicene" (after the council) categories, with the former often considered particularly important (though many later fathers are highly revered as well). Even more revered among them are the Apostolic fathers, fathers of the 1st and 2nd centuries who are believed to have personally known some of the apostles.
The writings of the fathers are often used as a reference point for determining what is considered orthodox, with authoritative documents like the Catechism of the Catholic Church making heavy use of their writings.