In OpenStreetMap, a feature is a physical element in the landscape that can be mapped. This can include both natural and man made objects, but OpenStreetMap is all about mapping real and current features of the real world. We don't include opinionated data like ratings, historical or hypothetical features.
The majority of features can be represented within OpenStreetMap as a single element (node/way/relation) with either a single tag, or a small number of tags to classify it. For example a path with a single tag for classification (e.g. highway=footway), and perhaps a name by using name=*. But since it's a worldwide, inclusive map, there are many unusual things in OpenStreetMap and we have ways to describe nearly all of them with tags.
See Map features for a list of common features and their tags. This list links through to documentation pages of the wiki with a "Tag:" or "Key:" prefix. You will also end up on such pages by simply searching the wiki. In most cases we have been able to agree upon simple intuitive tag names to represent features, but the documentation also gives explanations, definitions, and synonyms, as well as additional tags to achieve sub-classifications.
A "feature page" on the wiki is a description of a real world feature (or often a large scale umbrella concept) for which we have many related tags, and where it as been decided that extra documentation is warranted.
See Category:Features for an alphabetically sorted index of feature pages.