What's the difference between a process and a thread?

A process is a running program with its own resources and memory space. A thread is one path of execution inside a process. A process can have many threads, which usually share the process's memory. Separate processes usually have to pass data to each other through a dedicated channel.

In Node, the main JavaScript code usually runs on the one thread of a given event loop, but the Node process as a whole also uses other threads. So “Node is single-threaded” is a shorthand that can mislead without an explanation. A heavy computation can be moved to a worker thread or another process.