WP Official Site - www.wordpress.org
Get everything about WP, - the latest version of WP software download, themes, plugins, and the knowledgebase documentation. All is "both free and priceless at the same time" as bragged on this site.
WORDPRESS.COM - www.wordpress.com
The hosted blog service powered by WP software. It offers a free account which is too limited to be useful. The paid plans cost $3.99 (Personal), $8.25 (Premium), and $24.92 (Business) per month (rate as Sept 2017). The plans are cloud-based SaaS similar to Google Blogger which is totally free.
Automattic - www.automattic.com
Automattic owns WordPress and a few other web systems. WP is the flagship product. The company name has an extra "t" in the word automatic, which is for "matt." Matt is the founder of Automattic.
Matt Mullenweg - https://ma.tt/
Matt established Automattic and was the founding developer of WordPress.
MySQL - www.mysql.com
The open source database software, now owned by database giant Oracle, is the default database for WP.
PHP - www.php.net
PHP is the scripting language for Wordpress. Some PHP knowledge is required for WP customization.
Akismet - https://akismet.com/
Akismet (owned by Automattic) is an advanced hosted anti-spam service aimed at thwarting the underbelly of the web. It is included in Wordpress by default. Make sure it is turned on and configured properly.
WordCamp - www.wordcamp.org
WordCamps are casual, locally-organized conferences covering everything related to WordPress.
Wordpress TV - https://wordpress.tv/
Official video recordings of WP events.