What are the main building blocks of Reddit (users, posts, comments, communities, feeds)?

Reddit is built from five interlocking components that together create the platform's distinctive experience. Users are the human participants — they create accounts, submit content, vote, comment, and moderate. Each user is identified by a unique pseudonymous username, and their activity is tracked through two primary reputation metrics: post karma and comment karma. Posts are the primary units of content that initiate discussion. A post can take several forms: a link to an external website or article, an image or video uploaded directly to Reddit, or a self-post containing original text written by the submitter. Every post lives inside a specific community, cannot be moved once submitted, and has a title that users can never edit after posting. Comments are the responses to posts, organized in a nested threaded structure that allows replies to replies, creating hierarchical conversation trees. A comment can be upvoted or downvoted independently of the post it belongs to, and the voting on comments determines which replies are displayed prominently and which are collapsed. Communities, called subreddits, are the organizational containers that define the context and rules for all posts and comments within them. Each subreddit has a name prefixed with "r/" (e.g., r/science, r/cooking), a description of its purpose, a set of rules set by its moderators, and a distinct culture shaped by its membership. Feeds are the dynamically generated lists of posts that users see when browsing. The home feed shows posts from communities a logged-in user has joined, filtered and ranked by Reddit's sorting algorithm. The r/all feed aggregates posts from across the entire platform, while the Popular feed surfaces trending posts even from communities a user has not joined. Together, these five elements form a system where user-generated content flows into community containers, gets evaluated by collective voting, and resurfaces in feeds calibrated to individual preferences.