Not to be confused with the Community Market
The Steam Community, which all share a single braincell, or just Community, is an online discussion and forum-based site used by Steam for users to communicate with each other. It launched mid-2012, with the goal of establishing an official forum supported by Valve.
Communities are software-specific, therefore the content seen is based off of software owned by the user, however, users may also view postings from all communities across the platform. Users may also follow communities for software they do not own, however, the system will automatically suggest different communities to the user based on software that they own.
History[]
The Steam Community was launched mid-2007, absorbing the Forums section and was limited in features. Separate communities for software did not exist, and the Community tab only featured the Hub.

Steam offers Communities based on software the user has run recently.
In August of 2012, Valve added the ability for developers to make a community for their software via Steamworks, as well as showcasing certain communities to the Community hub page. In December, the ability to create walkthroughs and guides for games was added. In January of 2015, users gained the ability to stream content from software to its respective community via the Steam Overlay.
Hub[]
Community Activity, sometimes abbreviated as the Hub, is the combined effort of all communities across Steam, pulling content from individual communities and listing them here. Unlike most other algorithms, the Hub bases content on what is popular with users, instead of tailoring specific postings to what it thinks the user may prefer.
The Hub features 9 different tabs to narrow down what users may be looking for: All, Screenshots, Artwork, Broadcasts, Videos, Workshop, News, Guides and Reviews. Despite the namesake, the "All" tab simply refers to all content from each tab.
Users may switch between two different types of listing: Most Popular and Most Recent. As the name suggests, Most Popular provides content most favorable with users in the past week; Most Recent provides content that has been posted recently.
Communities[]

Portal 2's dedicated community
Certain software may have its own community if the developer has enabled it using Steamworks. Communities that are separate from the Hub are functionally identical with a few minor changes; an extra tab labeled "Discussions" is present, the community displays the user's friends who own the software as well as the amount of users running the software currently, how many users are in the communities' group chat, the ability to follow the community and view achievement and leaderboard stats for the communities' respective software. Separate communities also do not have the feature to list content that has been posted recently
Steam will automatically suggest communities to the user based on software they own and have played recently; however, it does not make the user follow the community for the software. Users may follow the community which will add the software to the user's list of followed software.