A VPS is a virtual private server connected to the internet that is running on a larger, real server.
Webhosting is renting (space on) a computer connected to the internet, on which you can run a website.
WordPress.com is WordPress as a Service. At WordPress.org you can download WordPress to use at your own hosting.
CDN is an abbreviation for Content Delivery Network and it does exactly what its name says: it delivers content over a network of servers.
WordPress is a so called Content Management System. With WordPress, you can easily build a website and add content yourself.
Caching is a technique used to store data in a temporary storage area, called a cache, so that it can be accessed more quickly in the future.
Transactional email is sent automatically as a result of a transaction on usually an app or website.