{tikz-styles} are predefined sets of options that organize how graphics elements are drawn. It is thus possible to program specific styles and reuse the styles on different objects. The {tikz-styles} tag can be used in parallel with {tikz-pgf}.
Questions tagged with tikz-styles are about managing styles within the TikZ/PGF graphics framework (tag: tikz-pgf). Both tags are possible on the same question.
In addition to TikZ-style examples in questions on this site, chapter 2.8 of the TikZ/PGF manual provides more insights. TeXamples.net shows examples of what can be done with TikZ-styles.
Additional overviews are:
