Source code

Is a collection of instructions and statements written in a programming language that can be read and understood by humans. It serves as the foundational blueprint for software programs, applications, or systems. Source code is typically written using a high-level programming language (such as Python, Java, C++, etc.) and may later be compiled or interpreted

Features of source code:

Readability: Source code should be easy to read and understand for other developers.
Comments: Source code may contain comments that explain what each piece of code does.
Structure: Source code may have a logical structure that makes it easy to find and understand individual parts of the code.
Syntax: Source code should conform to the syntax of the programming language in which it is written.

There are many source code licenses that define the terms of use, modification, and distribution of software. Here are some of the more popular source code licenses:

GPL (GNU General Public License): One of the most popular source code licenses, which requires that all derivative works must also be released under the GPL.
LGPL (GNU Lesser General Public License): A less restrictive version of the GPL that allows libraries and modules to be used in proprietary software.
MIT License: A very permissive license that allows you to use, modify, and distribute the software without restrictions, except that you must keep a copy of the license and the copyright notice included.
Apache License: A license that allows you to use, modify, and distribute the software without restrictions, except that you must keep a copy of the license and the copyright notice included.
BSD License: A license that allows you to use, modify, and distribute the software without restrictions, except that you must keep a copy of the license and the copyright notice included.
Mozilla Public License: A license that allows you to use, modify, and distribute the software without restriction, except that you must retain a copy of the license and copyright notice.
Eclipse Public License: A license that permits you to use, modify, and distribute the software without restriction, except that you must retain a copy of the license and copyright notice.
AGPL (Affero General Public License): A license that requires that any derivative works be released under the AGPL, and that any changes must be made available to everyone.
Unlicense: A license that permits you to use, modify, and distribute the software without restriction, without retaining a copy of the license and copyright notice.
CC0 (Creative Commons Zero): A license that permits you to use, modify, and distribute the software without restriction, without retaining a copy of the license and copyright notice.

This is not a complete list, and there are many other source code licenses available. Examples of open source code:

Linux: An operating system whose source code is available to everyone and can be used, modified, and redistributed free of charge.
Apache: A web server whose source code is available to everyone and can be used, modified, and redistributed free of charge.
Mozilla Firefox: A web browser whose source code is available to everyone and can be used, modified, and redistributed free of charge.

Examples of closed source code:

Windows: An operating system whose source code is not available to everyone and can only be used with permission from Microsoft.
Adobe Photoshop: A graphics editor whose source code is not available to everyone and can only be used with permission from Adobe.
Microsoft Office: An office suite whose source code is not available to everyone and can only be used with permission from Microsoft.