A copyright license will prevent everyone except the author from copying, distributing, modifying or selling the material (be it code, binary or anything else) irrespective of whether it is available publicly or not.
The author can (or has to) explicitly grant the right to copy or sell to select entities (companies or individuals).
An example would be Microsoft Windows. You cannot copy, modify or re-sell Windows under a different brand. But selected computer manufacturers have explicit permission to slightly modify Windows before shipping.
You may have seen certain PC vendors install their own software, change the wallpaper, change system policies, etc. Before selling the computer with Windows in it.
Copyleft licenses freely allow copying, distributing, modifying, selling of material under certain conditions. The exact conditions depend on the license being used.
A lot of open-source evangelists use a Copyleft license, because a Copyleft license forces the software using it to also have the same license.
For example, if you've built a software consisting of 10,000 lines of code with the intention of using a Copyright license but used a library (a small piece of code written by someone else to do something specific) of only 10 lines of code licensed under a Copyleft license, then you're forced to release the entire project under a Copyleft license 🙌
Due to this viral nature of Copyleft licenses, corporate companies avoid using several popular, tried & tested libraries & frameworks that are licensed under a Copyleft license.
Permissive licenses basically have no restrictions 🙌
The biggest difference being, you can use a software under a permissive license and you're not forced to release your software under the same license.
All you need to do is give acknowledgement to the software developer by including a copy of the license document.
What is the Unlicense? 🤔
Unlicense also commonly known as Public Domain is a license that allows anyone to do anything without any restrictions.
Software released under this Unlicense comes with no warranty. You're completely on your own when using materials under Public Domain.
🚨 Having No License
A project having no license has all of it's rights reserved to the author.
You can only view the source code but nothing else, that too only if it is publicly available (source-available).
An explicit permission is required from the author (copyright holder) to do anything to that codebase.
✨ What is Open-source?
In order for a project to be called Open-source, it has to satisfy the below requirements 👇