The miriad of different styles for bibliography and reference layouts can be, in the main, classified into three main “families”: the ‘label’ family, where references are denoted with label, usually in [square parenthesis]; the ‘author-year’ family (Adorno, 1978); and the ‘footnote’ family. Although there are arguments for each of those families, the choice between them is, in the last analysis, decided by tradition: different disciplines have come to adopt different styles, and new generations of authors will naturally follow the uses of their predecessors (which are enforced also institutionally, for example with journal guidelines).
LaTeX itself (with BibTeX) is designed toward the label family. Some packages, like cite, provide extra utilities in that family. The other two families are reflected in the LaTeX world by special packages. The author-year family is well illustrated by packages such as harvard, natbib, and achicago. Footnote-style references are implemented by the package opcit. A basic description of these packages follows. I will spend relatively longer with opcit, that is comparatively recent (2002) and the one I know best. A new release of it, moreover, is planned for the summer.