WordPress Trac: This is our bug tracker and project management tool, where the code happens. We track bugs, enhancements, and tasks here. SVN actions are deeply integrated into Trac, including creating patches, and commits by core developers. Trac is for discussing code. Philosophical issues and questions over implementation of a potential future feature do not belong on Trac. Trac is located at https://core.trac.wordpress.org/.
WordPress SVN and Git: The Subversion code repository is where the code “lives”, and is located at https://develop.svn.wordpress.org/. There is also a Git mirror available at git://develop.git.wordpress.org/. This is the core development repository. After a build process, commits to “develop” are synced to the traditional “core” repository, located at https://core.svn.wordpress.org/ (git://core.git.wordpress.org).
Slack: The #core channel on wordpress.slack.com is our place for real-time discussion. It primarily serves as the venue for our weekly developer meeting, but is also a general discussion channel for WordPress core development. Slack has replaced IRC as our primary public communication channel. To learn how to join Slack with your free WordPress.org user account, visit chat.wordpress.org. The channel is open to anyone with a WordPress.org account, but the majority of chatter, especially during our weekly project meeting, comes from core contributors. Often, a conversation for a Trac ticket will be pulled into Slack, with the consensus later posted to the ticket. Bugs and questions of implementation will often be hashed out in Slack before ending up on Trac. Many contributors idle here. The channel is logged and searchable. You can connect to the channel through the fully-featured web app, a native Mac app, and iOS and Android apps. There are also dozens more channels on the WordPress Slack organization, in addition to #core.
WordPress Blog: The WordPress Blog is a source of official announcements and news for the users of WordPress. The core team uses this blog to announce releases and initiatives. The blog feed appears in the dashboard of WordPress installs. The WordPress blog is located at https://wordpress.org/news/.
make/training: The blog for the WordPress Training contributor sub-group, part of Make/Community.
make/ui: The blog for the UI team. Much of the UI work is done under the umbrella of core, but make/ui is used for discussion of user testing and design issues.
Mailing Lists: WordPress leverages numerous mailing lists like most open source projects, but as a secondary tool. Patches are posted to Trac, rather than to any mailing list, and discussions on the mailing lists are often better suited in another venue. For example, wp-hackers was used for core development discussions years ago, but now these discussions will occur in IRC, on Trac, and on make/core. The list currently has a rather poor signal/noise ratio, but is still a source of good information and discussion when other venues might not be ideal. Additional mailing list information is available on the WordPress.org Codex.
There are some important mailing lists for those who wish to follow core development:
wp-svn: An announcement list of every commit to the WordPress codebase, which includes both the commit message and the actual patch of changes. Volume: There were more than 4,500 changes to WordPress in 2010, an average of about 10-15 per day. This fluctuates wildly depending on the phase of the development cycle, from 50 or more in a busy day to only a handful during a slow week.
wp-trac: An announcement list of every comment to Trac. Truly the WordPress firehose, there were more than 20,000 comments posted to Trac in 2010.
wp-testers: A mailing list for developers testing the current alpha, beta, or release candidate of WordPress. This list is typically dormant until the first beta release. Closely watched by core developers, this is a great way for developers to post questions or potential issues that can then be addressed by those familiar with the codebase and the changes that went into each release. The list has seen a decline in traffic over the years, with more individuals opting for the Alpha/Beta support forum or Trac.
wp-unit-tests: An announcement list of commits to the WordPress tests repository, and comments to the tests Trac. If you are subscribed to both wp-svn and wp-trac, you’ll already get these, but if you’re interested specifically in unit testing, you’ll want to follow along here.