Raspbian Update & Upgrade
A raspberry pi project has four major components
The Operating System (OS) (examples are: raspberry pi OS (nee raspbian), OSMC, and diet-pi)
The kernel raspberry pi OS runs on top of
The packages installed to enable your application
The application code
Linux operating systems have two kinds of upgrades:
Upgrade the current OS and its packages
Upgrade to the next version of the OS and kernel
The first one might be done multiple times over a couple of years. The reason to update and upgrade is to keep security patches up to date, and possibly fix performance or other issues the application might be having.
The second is called a dist-upgrade and is done once every one or two years. Some OSs should only be upgraded using a dist-upgrade (e.g., OSMC)
The table below shows debian release dates, and End-of-Life (EOL) dates. A dist should not be used after th4e EOL date.