The April even-numbered releases (14, 16, 18, etc.) are stable and receive long-term support (LTS) updates for three years.
The October odd-numbered release (15, 17, 19, etc.) are more experimental and updates often end after one year.
In general, you should opt for the even-numbered LTS version unless you require a specific feature in an experimental release and intend to upgrade later. That said, Node.js 20 is new and the website advises you continue with version 18 while the development team fixes any late-breaking issues.
Historically, Node.js has been a minimal runtime so developers could choose what tools and modules they required. Running code tests required a third-party module such as Mocha, AVA, or Jest. While this resulted in plenty of choices, it can be difficult to make the best decision, and switching tools may not be easy.