From Program to Process

A program is just static code somewhere on your computer; A process on the other hand is a program that is currently being executed, which means it has it's somewhere in memory (or outside; see paging).

A process can be in three different states, which are: Running, Blocked, or Ready.

Running: The process has currently access to the CPU. Blocked: The process can't continue execution until some resource or information becomes available (waiting on data from I/O-Device for example). Ready: The process can continue (or start), and is waiting for the CPU to be freed up.

Threads

Threads are a lightweight subsection of a process which has it's own control flow. This could be something like saving a file every 10 minutes (if edited), or checking if their are any new email in your inbox.

It's important that this subsection is somewhat independent.