Wikipedia is a compendium of the world's knowledge. If you know what you are looking for, type it into Wikipedia's search box. If, however, you need a bird's eye view of what Wikipedia has to offer, see its main contents pages below, which in turn list more specific pages.
Culture – encompasses the social behavior and norms found in human societies, as well as the knowledge, beliefs, arts, laws, customs, capabilities and habits of the individuals in these groups.
Geography – field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets.
Health – state of physical, mental and social well-being.
History – the past as it is described in written documents, and the study thereof.
Human activities – the various activities done by people. For instance, it includes leisure, entertainment, industry, recreation, war, and exercise.
Mathematics – the study of topics such as quantity (numbers), structure, space, and change. It evolved through the use of abstraction and logical reasoning, from counting, calculation, measurement, and the systematic study of the shapes and motions of physical objects.
Natural science – branch of science concerned with the description, prediction, and understanding of natural phenomena, based on empirical evidence from observation and experimentation.
People – plurality of persons considered as a whole, as is the case with an ethnic group or nation.
Philosophy – study of general and fundamental questions about existence, knowledge, values, reason, mind, and language.
Reference works – compendiums of information, usually of a specific type, compiled in a book for ease of reference. That is, the information is intended to be quickly found when needed.
Religions – social-cultural systems of designated behaviors and practices, morals, worldviews, texts, sanctified places, prophecies, ethics, or organizations, that relates humanity to supernatural, transcendental, or spiritual elements.
Society – group of individuals involved in persistent social interaction, or a large social group sharing the same geographical or social territory, typically subject to the same political authority and dominant cultural expectations. Societies are characterized by patterns of relationships (social relations) between individuals who share a distinctive culture and institutions; a given society may be described as the sum/total of such relationships among its constituent of members.
Technology – the sum of techniques, skills, methods, and processes used in the production of goods or services or in the accomplishment of objectives, such as scientific investigation.
Vital articles are lists of subjects for which the English Wikipedia should have corresponding high-quality articles. They serve as centralized watchlists to track the quality status of Wikipedia's most important articles and to give ors guidance on which articles to prioritize for improvement.
Outline pages have trees of topics in an outline format, which in turn are linked to further outlines and articles providing more detail. Outlines show how important subtopics relate to each other based on how they are arranged in the tree, and they are useful as a more condensed, non-prose alternative to overview articles.
Wikipedia:Contents/Outlines is a comprehensive list of "Outline of __" pages, organized by subject. It is itself an outline, that links (almost) exclusively to other outlines.
Outline of academic disciplines covers subjects studied in college or university and provides links to prose overview articles and their corresponding outlines.
Outline of knowledge is the top-level outline, its subject being the broadest one of all. It is the ancestor of all other outlines, and they branch out from it, in successive levels.
Wikipedia's collection of category pages is a classified index system. It is automatically generated from category tags at the bottoms of articles and most other pages. Nearly all of the articles available so far on the website can be found through these subject indexes.
If you are simply looking to browse articles by topic, there are two top-level pages to choose from: