Before talking about processes, we must first define what a program is. In short, a program is a passive collection of instructions for the server to run.

A process is the active execution of those instructions. The basic flow when developing is:

write code (program) 
  > operating system compiles code to binary 
    > run the binary (process)

A single program can be run multiple times at the same time, thus creating multiple processes.

Processes are represented by process identifiers (PID), a unique numeric value. (These numeric values can be recycled, so two processes could have the same PID, just not at the same time.)

Processes can be short or long-lived.

Parts of a process

When a program becomes a process, the running process can be divided into four parts:

• Stack: contains the temporary data, like method/function parameters, return addresses, local variables
• Heap: dynamically allocated memory to a process during runtime
• Data: global and static variables
• Text: compiled program code

Types of processes

Foreground: A foreground process is initialized and controlled through a terminal session. A user starts these, not the system.

Background: Not connected to a terminal and doesn't expect any user input. A daemon is a type of background process that starts at system startup and continues to run forever as a service.

Processes can also be divided into Parent and Child processes. Child processes are created by a parent process at runtime.

Process states

Processes can have a number of possible states. The names for these states vary by operating system, but the following five are pretty standard.

• New: process is being created
• Ready: process has all resources available that it needs to run, but CPU is not currently working on this process's instructions
• Running: CPU is working on this process's instructions
• Waiting: process can't run at the moment because it's waiting for some resource to become available or an event to occur (keyboard input, disk access request, a child process to finish)
• Terminated: process has completed